So I am running a game for people who were brand spanking new to D&D. 

So - I said, "For starters, only PHB stuff is available in terms of classes, races, and spells."

That way there was no sense of just being overwhelmed by the amount of choices that were present (with new sub classes and spells).

And the idea was - the "magic" of the world was trapped in a sphere. The party would eventually learn of this - and need to break the sphere to release the magic.

Now, the idea being - it'd be awhile until they got to this point. We play every 2 weeks - so I figured a year, and see how they are - we lost two players (one got a new job, and the other was his friend, so he didn't know us and decided to drop). Next session, recruited a new player who had never played. So a step back. Following session, another new player. Also never played before. So a year comes, and some are still not super grasping their characters, as they level. So year two - they learn of the sphere, learn they need a special hammer forged by dwarves (they'd already befriended said dwarves and recovered the hammer that'd be stolen by Drow and helped defeat a mad Drow priestess). So they return to the dwarves, get the hammer, and return - enter a pyramid, fight a greater mummy - uncover the sphere - and shatter it.

The world is filled with white light.

I allowed each player at that time - to be able to change their sub class (not their class, but whatever sub class they picked) - able to pick with all the books I had unlocked here on D&D Beyond - as the gods bestowed new knowledge in their minds.

Worked out great.

Allowed me to get them slowly introduced into D&D without being overbearing with all the options, and created a very fun quest and heroic moments.

I have 2: 

Tales of Moved Sands

A small desert world where the magical Oasis’s make life somehow possible. The party goes around completing quests until the BBEG Crinin Tornin the prince of the capital of the world burns down all the oasis’s and cause the entire world to flee into the frozen wilderness.

Vines Reveal Peoms

a world covered in lush vines and forests alike. But something is causing the vines that protect the trees and lives to burn down, turning the world into a desert wasteland.

My campaign setting has two major villains, a lich and an avatar of Talos. Without going too heavy into world structure, I have a pantheon of 14 deities (1 being the creator of the world who chooses to be completely disconnected from it) and the pantheon has been magically sealed off from directly interacting with the world (the lich did it). Some of the gods and goddesses already had avatars (like Talos), and some chose avatars after they were sealed off. And then there are a few oracles peppered in. 

I recently had an idea of making my lich kind of an anti-hero? Kinda? I watched a dungeon dudes video about liches and they mentioned having a lich prepare the world for a more powerful threat. I didn’t do exactly that, the lich is still evil and bitter over some events that happened to him in life. He wants to use the power my players will gain during the campaign to go take out Talos and his avatar after being used as a punching bag by Talos centuries before.

I originally wanted him to be a one-off villain that has been around for much of history but hadn’t interacted much with the world. Now he’s responsible for a lot. Like a lot a lot. And has several phylacteries, leaving the possibility for him to be a threat again in a future time in the world. 

I guess I just wanted somewhere to share what I was thinking of (my wife and some friends are my players). But if anyone has any questions or feedback regarding this idea, I’d love to hear it!

Here’s a small section of of larger concept my table is about to embark on:

One of the deadliest pirates of the modern age escaped during a massive prison break from Revel’s End; and has since met up with his old crew; and has rapidly resumed his crusade along the Sword Coast; upsetting all naval trade along the coast of the continent.

Previous pirate captains have been forced to submit; or die…often gruesomely…this pirate lord now commands a collective naval force which rivals any singular city; and of course politics are preventing individual cities from uniting their forces against it…some are even suspected of collaborating with the pirates to dominate their competition.

Unbeknownst to all, this pirate lord is a devotee of Umberlee, the sea goddess of destruction…and every sunken offering to her is slowly bolstering her strength; threatening to widen a deep-sea portal to the Elemental Plane of Water…once this pirate lord has amassed enough tribute; they plan to use this power to drown the continent in watery destruction, elevating Umberlee to a major deity.

I'm working on a pretty grand campaign idea at the moment. Its a take on the "Library of Fate" classic. My party, currently in Faerun, find an ancient magical book. When they examine it they find themselves magically pulled in to its pages and find themselves in Eberron. After some adventures there and some clues about the book, they find another book and find themselves pulled in to Ravnica. I'll have them go through a few different worlds like that, picking up clues that lead them to the Library of Fate, which is an infinite library containing a book for every world in the multiverse. The library was attacked and a bunch of volumes stolen, they are charged by the Librarian Eternal to track down the remaining books and find and stop whomever it was that stole them in the first place (haven't decided exactly who that will be yet, but I'm thinking some kind of trickster god)

I created a campaign setting that is a dark alternative to "The Frog Prince", where a bullywug, with the backing of several magic users and alliances with dragons, bewitches a princess into marrying him and overthrows the current king.  This brings about a dark age, where warm-blooded creatures are treated as second class citizens, and the Iron Flipper of the King has spread throughout the land.

I originally wrote it as just a silly backdrop to reskin Raptorheist's "King's Gambit" one shot for my players.  Lo and behold, we are now in a full-fledged campaign.

Here's the player's guide I made with Homebrewery if anyone is interested.

Some campaigns I've kicked around but might never do, because I think I will run a published module for my next, and by the time I homebrew again I'll probably have a shinier idea:

1. A low-magic campaign set in a vaguely post- Bronze Age Collapse world.

2. A continuation of an alternate ending (still maybe possible, but unlikely) of my current campaign, where the tier 3 party escapes the destruction of the earth by ascending a huge tower to the sky. The rest is vaguely spelljammer and/or planeswalker. I thought even about running it as an episodic campaign where we switch off as DMs. A planet / plane of the week kind of thing.

Here's a generic 5 adventure campaign outline:

Adventure 1: The players are introduced to their characters in their ordinary world, whether it be a bustling city or a quiet village. They receive a call to adventure from a mysterious figure, who promises a reward if they complete a task. The players meet their mentor and prepare to leave their ordinary world, but must first face a reluctant guardian who challenges their resolve.

Adventure 2:  The players meet a supernatural ally who provides them with assistance, whether it be a powerful wizard or a helpful faerie. They must learn to use their new abilities and confront a major challenge, perhaps a dangerous monster or a treacherous terrain. The players gain new knowledge or insight from their supernatural aid, but must also face a series of tests and encounter allies and enemies.

Adventure 3:  The players approach a place of great danger or power, such as a haunted castle or a deep underground cavern. They face their greatest fears and doubts, and must overcome them to proceed. The players find a way through the obstacle and gain new knowledge or insight, but also face a major ordeal or challenge, often a life-or-death situation.

Adventure 4:  The players receive a reward or boon for their efforts, perhaps a valuable artifact or a powerful weapon. They begin the journey back to their ordinary world, but face new challenges and setbacks along the way, such as a powerful adversary or a difficult terrain.

Adventure 5: The players return to their ordinary world, but it has changed. They face a final challenge or confrontation, perhaps with the same mysterious figure who first called them to adventure. The players achieve a new balance or integration in their lives, perhaps as respected heroes or powerful rulers of a kingdom.

Here's a 30 step outline in 10 adventures, where each step can be a session:

Adventure 1
The party is introduced in their ordinary world.
The party receives a call to adventure.
The party meets their mentor and prepares to leave their ordinary world.

Adventure 2
The party is reluctant to accept the challenge for some reason.
The party meets a guardian who challenges their resolve.
The party overcomes their reluctance and decides to accept the call.

Adventure 3
The party meets a supernatural ally who provides them with assistance.
The party learns to use their new abilities and confronts a major challenge.
The party gains new knowledge or insight from their supernatural aid.

Adventure 4
The party crosses the threshold into a new world and faces a challenge.
The party meets a threshold guardian who tests their resolve.
The party overcomes the challenge and gains a new sense of purpose.

Adenture 5
The party faces a major setback or crisis that plunges them into darkness.
The party must confront their deepest fears and doubts to escape the crisis.
The party emerges from the darkness with new knowledge or power.

Adventure 6
The party faces a series of tests and encounters allies and enemies.
The party learns from their allies and gains new skills or knowledge.
The party faces a major challenge or setback and must regroup and reconsider their approach.

Adventure 7
The party approaches a place of great danger or power.
The party faces their greatest fears and doubts, and must overcome them to proceed.
The party finds a way through the obstacle and gains new knowledge or insight.

Adventure 8
The party faces a major ordeal or challenge, often a life-or-death situation.
The party overcomes the ordeal and receives a reward or boon.
The party faces a setback or loss but gains a new sense of purpose or direction.

Adventure 9
The party receives a reward or boon for their efforts.
The party begins the journey back to their ordinary world.
The party faces new challenges and setbacks on their journey back.

Adventure 10:
The party returns to their ordinary world, but it has changed.
The party faces a final challenge or confrontation.
The party achieves a new balance or integration in their life.

And, because what the hell as everyone files in for our game tonight:

Adventure 1: Fantasy Adventure - The party is introduced in their ordinary world, living in a peaceful village on the edge of an enchanted forest biome. One day, they receive a call to adventure from a messenger of the Queen of the Forest, who requests their help in stopping an evil sorcerer who seeks to destroy the forest.

Adventure 2: Fantasy Comedy - The party is initially reluctant to accept the challenge, but after encountering a bumbling wizard who tries to dissuade them, they decide to take on the task of saving the forest.

Adventure 3: Fantasy Action - The party meets a centaur ally who provides them with assistance, including a magical bow and arrows. They use their new abilities to confront the sorcerer's minions and eventually face the sorcerer himself in a major battle.

Adventure 4: Fantasy Western - The party crosses the threshold into a new world, a wild-west inspired town built on the edge of the enchanted forest. They face a challenge from a gang of bandits who are in league with the sorcerer and must prove themselves in a showdown.

Adventure 5: Fantasy Horror - The party faces a major setback when the sorcerer unleashes a powerful curse on the forest, causing the trees to wither and die, and the creatures within to turn malevolent. The party must confront their fears and doubts to escape the dark forest and find a way to lift the curse.

Adventure 6: Fantasy Gangster - The party faces a series of tests and encounters allies and enemies, including a group of smugglers who offer to help them cross the pirate-infested sea to reach the sorcerer's stronghold.

Adventure 7: Fantasy Drama - The party approaches the sorcerer's stronghold, a grand castle perched atop a mountain. They face their greatest fears and doubts as they climb the treacherous terrain and face the castle's guardians.

Adventure 8: Epic Fantasy - The party faces a major ordeal or challenge as they confront the sorcerer in a battle of magic and skill. Despite facing setbacks and losses, they manage to defeat the sorcerer and receive a magical boon from the Queen of the Forest.

Adventure 9: Historical Fantasy - The party begins the journey back to their ordinary world, but are waylaid by a group of knights who seek to claim the magical boon for themselves. They must use all their skills and knowledge to outwit the knights and continue on their journey.

Adventure 10: Fantasy Heist - The party returns to their village, only to find it changed by the aftermath of the sorcerer's curse. They must face a final challenge in stealing a magical artifact from the knights, which will restore the village and the forest to their former glory. With the artifact in hand, they achieve a new balance in their lives and become the protectors of the enchanted forest.

I had an inkling of an idea today that I might want to explore further in the future.

A world in which there are no gods, no devils, no angels, no celestials etc.Consquently Clerics are charlatans with no real power and the class is removed as a playable class. Paladins are deranged zealots and also removed as a playable class. Subclasses that derive their powers from celestial or devils (such as celestial or fiend warlocks) are removed also. I think I'd probably have to remove the Monk as well, but they might continue to exist as martial artists. Probably a bunch of races would be culled as well

Apart from the  Prime, Astral and Etheral planes only the four traditional elemental planes exist.

Magic still exists, and I might have to tweak some healing magic onto some other classes (like a wizard school for healing magic, beefing up the Druid class as healers).

I wonder if 5e mechanics would work without being misaligned, have to think about that

I've played in two different D&D 5e groups that ran the Princes of the Apocalypse adventure, and both DMs have tried to get creative in how to link the enemy activities together into a central plot, since that doesn't seem to be a thing as written. I've had an idea to run it with a pretty major twist of my own.

The conflicts and dungeon layouts would essentially be the same, but it would instead be set in the Final Fantasy 14 setting of Eorzea during the story of the Dawntrail expansion. The party would consist of a budding group of either new adventurers or the alliance's grand company recruits. They would initially be tasked to help out with small town troubles and discover the beast tribes causing trouble and trying to summon their gods. When they arrive just in time to witness the last group summoning their god, a new version of the Ultima Weapon will swoop in and absorb its power. The weapon's pilot, who the party would have met and worked with prior to this, would reveal themselves to be a Garlean engineer that still shared the ideals of Gaius Van Baelsar. This engineer had orchestrated and coordinated each of the tribes to goad them into conducting their summons so that the weapon could leech off of their power, and the final confrontation for the party would be to disable this new weapon.

If you haven't played the game up to at least the end of 2.0, then that explanation probably didn't make a lot of sense, which is the main reason I don't think I'll actually be running this concept for my current group. That, and I'm not sure I would have the patience to cut and modify that much of the adventure's identity to fit the setting.

I had an idea for a campaign where the entire party are devils who have good alignments and start a revolution against Asmodeus or do something small like making a trying to get a devil to be good.