So I am probably a minority in this, I think I am for a lot of DND things, but I like to impulsively write campaigns. There was probably a week that I got 4 campaign ideas. While I don't do details for all of them, I have wrote out backgrounds and or session 0's for most of my ideas. 
That's besides the point. I just want to see other people's campaign ideas, and thought a thread to share maybe not fully developed ideas freely would be cool. I think DMs Only is more appropriate than story and lore for this to.

I'll share a few:
Mythics Abyss - Somewhat based off Made in Abyss, where aberrations have gone missing through most of the continent, and there's an abyss that normally is filled with them. Mind flayers have actually been behind it the whole time, but the party doesn't know, and were hired by a scientist who wants to know more.
Rags Riches and Wishes - There was an ancient hero, wizard, and the tarrasque. The hero has slain the tarrasque, and the wizard has made the most powerful item in existence. A tablet that can grant any wish according to each base of alignment, and the story was lost to time. A woman had used the clone spell to create more tarrasque to destroy the world for praising the hero that killed a god creature. The only way to get rid of that many would be the tablet to wish them away.

1. Currently going through Forgotten Realms to introduce the new players to the basics of D&D and Fantasy and its iconic elements. Running Sunless Citadel, Forge of Fury into Red Hand of Doom. Then second part of Rise of Tiamat that gets padded by Storm Kings Thunder and lots of other stuff. They can basically traverse the entire continent so I got stuff ready for different planes, jungles of Chult, City of Thay etc. In this game the group gave up the deed to the Keep of Vraath in turn for ownership of another plot of land. Which is on top of an old dwarven ruin they had found in the grasslands. Building a keep on top of it and doing some secret excavations. However we turned that into a spin-off campaign for a group of other players with 1 player of the original group.

2. So the OG group hires upstart adventurers to clear out the ruins and will pay them. They travel to deal with Tiamat and devils while the spin-off group does this new thing. A large dungeon that'll eventually lead into the Underdark. Running some modules of the Princess of the Abyss and The City of the SpiderQueen and some other underground/demon themed content.

3. I have a Sci-Fantasy setting that gets expanded on through 1shots. Each 1shot is basically a group of Security Specialists from one of the big organizations/empires. Send to investigate,  hunt, kill or do whatever. Gradually expanding the setting until it is complete and lived in enough to turn it into a full fledged future campaign. The next one shot is about them having to board a derelict ship that came out of deep space, after having been lost for a long time. A la Warhammer. This ship now houses some Far Spawn creatures, the scientists turned into husks etc. Creepy stuff.

4. Waterworld style campaign that I'm building. Has aspects of One Piece, Pirates of the Caribbean and the book series Grim Company etc in it. Quite complex and will take a long time to be complete enough to play in.

You are not the only one who just comes up with campaign ideas. I haven’t really even started playing D&D again yet after many years of mostly doing MMOs and other computer based versions of RPGs, and not really anywhere near ready to start GMing again... and on top of that have no group of players to play with other than the group in which I will be a player, not a DM. But I am still doing things like world-building and coming up with campaign ideas.

One concept I am toying with right now is a D&D world set in Ancient Rome rather than the more typical medieval style. There are all kinds of interesting consequences of that, such as certain weapons had not been invented yet, certain government ideas were very different (there weren’t, inside the Empire, things like kings and dukes, but rather “Patricians”). And there was a huge untamed area outside the Empire’s borders that is perfect for orcs and trolls and the like.

1. Parts of the Lich Queen - Basically the premise of 2019 Hellboy. A cult is trying to gather the separate body parts of a powerful Lich Queen to resurrect her.

2. The party is sentenced to execution via a bottomless pit. Whether or not they're actually guilty of their crimes is some good RP material. But they reach the bottom unharmed and find a society of "executed people" living underground. Their mission is to make it back to the surface.

I use an old 2e campaign world called Mystara that I have been using since the early/mid ‘90’s. At this point (after over 25 years playing/DMing in the same world) I have accumulated so many personal additions to the world above and beyond the plethora of published materials for that world I would have no idea where to start.

1. A elder evil named gigginihap is coming back from his prison and he has made a bunch of mist that mind controls those that go in it.

2. A crazed mage used wish, gylph of warding, and a spell gem to cast a bunch of wishes at once and shred all the planar walls that keep out things like devils and elementals. The players have to go to each plane of existence and gather the essence of the wall from each one to rebuild it.

Hi

I've never actually DM'ed before but I want to have a go at writing my own adventure. 

Rather then setting it in a dungeon I'd like to set it in a haunted house. The house would have been owned by a secret society (explaining why there are traps and secret tunnels). The society found an artifact that weakens the curtain between the realm of the living and the dead. So I'd like to have all the monsters be kinds of undead that have crossed over, or former members of the society brought back. The team has to find the artifact and either bring it back to a patron or destroy it. 

Is there an undead adventure I could base it on?

The demons won the blood war. The angels and gods abandoned toril, not wanting to be killed by the demons. The Underdark is the top layer of the Abyss now, and the world is essentially a post-nuclear catastrophe similar to the Fallout games. Pools of demon ichor everywhere, monsters cover the continent, all dirt is gray and muddy, small civilizations survived, but large cities are in ruins. This campaign would probably take place around 5,000 DR, and everyone has forgotten the name of the world and call it Fliaote (fuh-lee-ow-teh), and demons dwell on land, obrynths in the Underdark (which is now flooded with Stygian water) and the oceans are foul, polluted by Aboleths and Illithids. Mind Flayers evolved webbed tentacle-like legs and gills to live underwater (they live mainly in lakes and seas), and all access to the world is blocked. 

too many to count

This might be a little controversial and would probably end up being too complicated or require too much railroading but what if the mission was a cross between Shrek and suicide squad. 

The PC's are all magical creatures that start in a concentration camp after the lord protector ordered all magical creatures rounded up. However, in order to make his territory a kingdom he has to marry a princess. Knights have been sent to acquire one but they have all failed. So, the lord protector has a wizard mark your players with a magical brand that will kill them if they don't do the mission. They are then sent out to rescue/kidnap a princess. Once they find her, they discover she is a powerful witch who can break their brand. The players then have three choices, help the princess get back to her kingdom to get allies, force the princess to come with them to exchange for their family members, or just try to sneak back without the princess and rescue their loved ones. 

A "rock opera" esque adventure. Haven't fleshed this out too much, but the idea is basically members of a rock/metal band get transported to a fantasy realm where they must go on an adventure to defeat some great evil. Every character starts with at least one level in Bard and everyone gets a magic item based on the instrument they play. The world is crafted on being a play-on-words of alot of classic rock songs and artists (for example, the kingdom is not ruled by a queen, but is ruled by Queen). The entire feel/setting of the world would be something like what youd imagine from power metal lyrics. 

Its a really fun idea I think, but as I said I have not yet put more thought into it 

here are my brain children (plz do not steal/"kidnap")

A female platinum dragonborn approaches the party in a pub (or on the road),and hires them to track down and slay some dragons. inside these dragons are orbs that the players can find,but not use,the PDB (platinum dragonborn) reveals that this item,and others like it,are the reason she hired the party. Assuming the party does not roll high enough to find out stuff sooner,they get to the end of the story having killed all the dragons the PDB has asked them to kill. At which time as she thanks the party,takes out all the orbes,swallows them,and as she absorbs their power, transforms into her true form:THE QUEEN OF EVIL DRAGONS: TIAMAT! Then the party,who would likely be weak from just fighting a dragon,are told this:"as a reward for bringing me back to full power,i give you this choice:leave with your lives ,or fight me and die."

this would likely take place some time after finishing rise of tiamat,where the party thinks they stoped her from comeing back,but she did,just powerless,with the orbs holding her lost power.

A monster that defeats a ally/player and drags their knocked out self away from the rest of the party to a different room or something and turns the player into one of them(or maybe some similar type of monster). like the player monster would have the same stats but maybe something extra cause of the change,and they fight the party till they die or roll high(15) on a will save to snap out of the mind control or whatever,then maybe they stay mutated for the rest of their life or maybe they could go on a quest for a magic cure/dispell magic/wish spell. (ONLY DO THIS WITH PLAYERS CONSENT!!!)

A man walks up to the party,talks about a cave/old temple (or whatever),becomes a temp party member,takes party to dungeon when they get to the end they are in a large chamber full of treasure ,the old man then reveals that he is a brass dragon in human form and tells the party "thanks",gives them one gold each and then tells them to go away(or maybe give more quests,if they think the party is "worthy",). (The party will  likely try to fight the dragon)

2 npcs walk up to party. One a poor looking old man wanting help with something strange on his farm,the other a young wealthy looking man in plate armour wanting help with "slaying a ferocious beast". The beast is a dog the party can have a chance to recruit and the farm thing turns into a epic quest to ... Do something epic.

A npc that the party meets at the start of the campaign and gives them quests and helps them sometimes and stuff. And when they get to the throne room of the BBEG the room is empty. But the npc walks up to the throne and sits down. And if anyone asks,he says that he is sitting in his throne. Then make up something from there. (l know this has likely been done before but whatever)

so what to you beyonders think of these? plz dont use these/post them elsewere without my ok.

The Spider's Web - After being arrested following a bar fight, the players are offered freedom in exchange for helping to solve a series of mysterious disappearances. Their investigation eventually leads them to a mysterious cult that worships the Drow goddess Lolth.

A Simple Favor - The players arrive in a small town on a stormy night. Needing directions to the inn, they approach a stranger who says that enough rooms for all of them will be very expensive, but he may be able to help. It turns out he's the duke and he can get the innkeeper to let them stay for free in exchange for helping him with a problem. Several months earlier, his wife was killed by a bandit who also stole one her most prized possessions, a large ruby. Eventually, they manage to apprehend the bandit and get the ruby back. The duke then reveals that the ruby has a pit fiend that he intended to make a pact with sealed inside it with the imprisonment spell and that he actually killed his wife himself because she sealed the pit fiend inside the ruby before he could make the pact. He also reveals that he intends to break the spell and release the pit fiend so he can complete the pact, which would grant him power and immortality in exchange for the death of everyone in town including the players.

I built my campaign world over 25 years as a dm. It started on a single continent in what is now 800 years in the past, and I built it out as needed. Now its 6 continents in a living world with hundreds of characters fleshed out, deep history from the many campaigns played on it, and 5 binders full of lore, history, and records of previous adventures. 

Don't try to come up with a whole world at once, you'll drive yourself crazy and it will be a sub standard mess, most of which your players will never see.  Build it out one piece at a time, as you need it, the way you want it. 

I've made a few, one of which I hope to publish on DMs Guild.

Ok so this campaign is set on a large island where native humans have found a way to coexist with the dinosaurs that live there. Herbivore dinosaurs are the only ones who live along side humans and have evolved to be able speak. The predators are unable to speak and are just normal dinos. 
Before you ask yes even some of the bigger ones can speak. The city that this civilization lives in is called Dinotopia. There crystals called Sunstones that protect it from the predators.

There are also humans who do not live along side dinos and hunt them instead. They are simply called Outsiders and there are several groups that follow their own quests to steal sunstones from the city and use them.

The players are people from modern society who’s plane crashed in the storm that surrounds the island and wash up on shore. And the adventure goes from there.

When I run Session Zero, I actually ask my players the style of campaign that they want to play in, and then create the campaign around that. It's really important that the DM is designing a game that the players want to be in - there's no point in running a dungeon crawl every session if the players would rather be mostly RP'ing in gothic ballrooms. My campaigns focus around a single storyline that starts at level 1, so it also helps the players to design characters that will make sense in the overarching storyline.

These were some suggestions that I gave the PCs last time:

• You are a special forces team, working at the forefront of an ongoing war, undergoing missions. Key game themes: action, clearly defined missions, small scale play as part of a larger war
• You have been hired to protect a bunch of archaeologists on some kind of excavation. What could go wrong? Nothing bad ever gets unearthed from old tombs. Key game themes: exploration, saving the world, epic calamity, structured narrative
• You are all related to, or otherwise serve, a child who is about to inherit a throne. It will definitely go well. Maybe one of you is in line to the throne. Key game themes: politics, alliances, going on the run, freeform choices with little direction
• You are all part of the same community (e.g. a village cut off from help), which stands on the brink of destruction, and must turn the tide. Key game themes: a single vital location, exploration, constructing a base, time-based incidents
• The political power in the region is bad for it. You all decide, for independent reasons, to work together to overthrow it. Key game themes: freedom fighting, being on the run, action, alliances. Freeform choices with little direction
• A new area has opened to explorers. You are able to go and explore differing parts, with a base to return to. Perhaps it is the ruins of an ancient civilisation. Key game themes: exploration, lost world, limited resources, survival. Freeform choices with little direction
• For some terrible reason, you have to serve an evil necromancer and now it’s time to help him take over the world. Gather up some bones, go complete an evil rite, destroy that church (the opposition will all be bad too. Everyone is bad). Key game themes: play as the bad guys, freeform choices with little direction

Some of my ideas revolve around a specific historical / mythological setting.

Late Bronze Age collapse

Martial classes are rare, because the tin trade routes have been cut. Anyone in possession of an existing bronze weapon is automatically an elite. A level 1 fighter can be a minor warlord. It's also low-magic, with homebrew reduced spell slots, but more powerful ones. Like you get 5th level spells at 5th level, but only one slot per level.

Post-Roman Britain

The setting of the original King Arthur tales like Y Gododdin and Culhwch and Olwen. Clerics are Roman Catholic priests. Monks are Irish hermits. Druids are Celtic druids. Knights of the Round Table are paladins. Picts are barbarians. The Holy Grail is a magic item but it's a stone that grants kingship.

A Lord of the Rings campaign where the party is four level 1 halflings and a level 5 ranger, but the ranger player doesn't attend the first few sessions. Instead he hears a recap of the sessions and has one-on-one sessions with the DM where he takes action to protect the party without their knowledge. After the party level up a few levels, the ranger joins the party.

Another non-standard narrative structure where everyone creates two characters, and we switch off every other session playing two different parties in different parts of the world. Gradually over the course of the early game some members of each party get killed off. After several sessions, the parties meet and merge.

After years of world building and coming up with epic campaign ideas I have really found I love just not doing that now. Campaign concepts for me run to 5-6 bullet points and then define any limitations my players need to know from that (there are no elves in my world, magic is scarce and magic users are hunted, etc). Then everything else comes from working with my Players. They tell me what they are playing and I work out how that fits in my world and let that shape my story. I work with them on back stories and ideas. I let them tell me the kind of campaign they want to play. We want to be pirates, we want to travel the planes. 

I also don’t write out long world histories or exposition, I define it on an adventure by adventure session by session basis drip feeding it to my players as we progress. It means I can change things, make new cool stuff up but also. Players don’t actually care what the name of a war 1000 years ago was unless they need to know it right now to solve something in the game. If you think about the best books, computer games and movies they don’t come with a 60 page handout defining the background of everything. You as a reader or player learn along with the character, or ideally get shown or in a last case told (I hate exposition in my games, I try and avoid ever having to exposition guy). 

So no I stopped writing out campaign ideas long ago. I remember I read a great quote I don’t remember who it was from. If you have an epic world and a long story arc pre defined With a beginning middle and end your players will follow you don’t have an rpg campaign, you have a novel and you should go and write it :). I think that’s a little harsh on some respects because I know people love world building but I think the sentiment is one all DMs should think about. Am I making a world or am I writing a novel. 

Quote from Scarloc_Stormcall >>

After years of world building and coming up with epic campaign ideas I have really found I love just not doing that now. Campaign concepts for me run to 5-6 bullet points and then define any limitations my players need to know from that (there are no elves in my world, magic is scarce and magic users are hunted, etc). Then everything else comes from working with my Players. They tell me what they are playing and I work out how that fits in my world and let that shape my story. I work with them on back stories and ideas. I let them tell me the kind of campaign they want to play. We want to be pirates, we want to travel the planes. 

I also don’t write out long world histories or exposition, I define it on an adventure by adventure session by session basis drip feeding it to my players as we progress. It means I can change things, make new cool stuff up but also. Players don’t actually care what the name of a war 1000 years ago was unless they need to know it right now to solve something in the game. If you think about the best books, computer games and movies they don’t come with a 60 page handout defining the background of everything. You as a reader or player learn along with the character, or ideally get shown or in a last case told (I hate exposition in my games, I try and avoid ever having to exposition guy). 

So no I stopped writing out campaign ideas long ago. I remember I read a great quote I don’t remember who it was from. If you have an epic world and a long story arc pre defined With a beginning middle and end your players will follow you don’t have an rpg campaign, you have a novel and you should go and write it :). I think that’s a little harsh on some respects because I know people love world building but I think the sentiment is one all DMs should think about. Am I making a world or am I writing a novel. 

 Yeah, this is how i'm creating my setting. I have a few locations l know they'll go to, and some ideas for quests for those locations, but other then that, its procedurally generated. (IE, other then the starting location, and the capital city of the country they start in, and some quests related to those locations, everything is made when it needs to be.)