First BIG Campaign

I'm doing my first large scale campaign. Full world building and everything done by me. But I'm having trouble thinking of how to start it. One idea I've had is starting off with the party dead in some way and make their first major quest regaining their bodies and figuring out what happened to them, who or what killed them. I'm just stuck trying to figure out a good way to do that, maybe even check out or make some homebrew subclasses that would work well whatever I can come up with for how they start.

Greetings LoordChungasss,

Welcome to the D&D Beyond Forums :)

Creating a full world, and building a full campaign (multiple long adventures strung together) can be very challenging and time consuming, but rewarding if everyone has a good time at your table.

As you did not state what books you own or have already read, or what experience you have with the game (playing or DM'ing), I'll start with some basic suggestions...

First off I'd read through the 2024 DMG, as it walks through being a DM and creating worlds in a much better manor than previous editions.

Additionally D&D released their Dungeons Master's Workbook of Worldbuilding back in the spring which might be helpful for you.

There are also tons of free resources online to help your worldbuilding like World Anvil and others.

Cheers!

I'd start with the relevant bits when you build your world first. The things that you plan for your players to actually interact with. Small/largish towns, sprinkled in an area of wilderness with some quests or whatever. Otherwise, you end up with a whole bunch of stuff your never gonna use off the bat, instead of the things you actually need to run the game.

You mentioned regaining their bodies. Is this like a mind swap, they're ghosts, or something else? There's actually this premade DnD campaign called Turn of Fortune's Wheel, where the players each have three characters, and every time one dies, another one appears within a few days. Eventually, they confront the main villain, and they reclaim their memories, a bunch of levels, and some magic items, before going off to fix the glitch that made all the chaos happen. That might be a bit of inspiration for you if you want to look it up.

First off: Who are you playing with? Generally if you are playing with newbies you DO NOT want to start introducing homebrew stuff until at least session 5. It can be extremely confusing to new players if you start them off with stuff not in any book and that they can't look up online. Personally, with new players I start them at level 1 and don't introduce homebrew stuff until level 5. 

Second off: What kind of campaign is it? Is this a on-rails narrative focused campaign? Or is it a open-world directionless sandbox? Something in between? You want to set up the player expectations right from the start.

Third: What is the tone you want to set? Is this a serious campaign with realistic consequence? Is this a more chill rule-of-cool campaign where the players can get away with all kinds of wacky hijinx? Is death to be considered a real thread in the campaign? If you start off the campaign with the players regaining their bodies after being dead, they aren't going to be worried about dying later on in the campaign. Is that something you want or not?

Note: these are not supposed to be leading questions, all these options are equally valid, but they are different types of game suitable for different types of characters and different types of players / styles of playing. 

My idea of it is they start as spirits of some sort, I also had the idea that they have to fight their bodies to regain them, like they're possessed by something and that's the way they have to come back. I think it can really extend the length of this if I make them slowly each regain their bodies as we progress through the campaign

Have you heard of the Crocked Moon DnD campaign. It starts off similarly and has a creepy theme. It might be worth looking into. 

I might be careful about making that intro extended... Starting as spirits and needing to get their bodies back sounds like it could be cool and fun! I might use the Ravenloft Survivors rule as the basis for that. It's a good way to do a low-powered and vulnerable character in a world of danger. 

But low-powered or not, it's likely that your players will want to get to play as their actual characters sooner rather than later. An extended adventure to get their bodies back, one which keeps them from actually getting to play as the barbarian or warlock or bard that they rolled up, is the kind of thing that can end up a lot more fun for the DM than the players. 

Maybe at least talk to them about that idea first. Make sure that this is something that sounds like fun to them too.

Starting off dead, and then coming back can be an interesting idea. Who brought them back? Why? Who killed them? Why? What are they going to do about it? That could certainly be the basis for a solid campaign. 
It’s the spirit thing that’s tripping me up here. It’s a little high concept, and those kinds of ideas often tend to collapse when the players start poking at them. And for a first campaign, it’s pretty ambitious. As others said, I wouldn’t start off with homebrew subclasses, particularly if they are newer players (which maybe they aren’t?)

But for this being your first go, I’d keep it more simple and stay away from the spirit angle. There’s a lot of moving parts in running a campaign, most of them you won’t even be able to anticipate. I would be very careful of adding in this extra wrinkle.

Here are some examples of how I've started my campaigns/one shots: 1) They're on a cruise ship attacked by pirates (naturally led to them being hired to hunt them down) 2) They're revived in an underground sealed chamber & forced to work for the Necromancer who revived them in a zombie apocalypse 3) they're part of a guild hired to hunt down a strange magical monster (led them into conflicts with mages who are illegal in this world) 4) they're shipwrecked in a strange new land & get up in a political war 5) they stumble across a corpse of guy who's recently died and now several people in town are hunting for them (it was the start to a mystery campaign) -> hope these give you some ideas.

Here are some specific thoughts on your own idea:

1. Spirits are complicated as others have said - one issue would be healing. Technically undead can't be healed. I'd rule it that the party healer can heal them (& if needs be grant one member the power to brew 'spirit potions')
2. I'd recommend just keep their classes - it'll be really complicated if they all act as ghosts with all the same abilities or if you're trying to do a lot of homebrew stuff as well
3. If they can regain their bodies, are they actually dead? 
4. You could have some villainous organisation running experiments on people and this spirit apparition is an unexpected (and unknown to the organisation) side effect => the PCs would have to start far away from their bodies (if you want to keep this long term, short term this could be the starting quest & after they regain their bodies they'll probably want to continue hunting down this organisation for justice or revenge or whatever). 
5. Assuming organisation, you'll want some goals for them - are they trying to build a 'better' humanoid? Create a vessel for their dark god to walk upon the material plane? Seeking to access the great magical power of the soul?
6. How it happened - 1) unexpected side effect, e.g. the experiments were conducted in a graveyard or some other place suffused in necromantic energy (maybe in the negative planes or next to a planar rift) & the natural energy mixing with the substances and tools used in the experiment generated fully corporeal spiritual versions of some subjects (ones with a certain unique gene maybe) 2) a deity/powerful entity doesn't like the experiments but the organisation is hidden from their sight; in their efforts to seek them out they made a tenuous connection with comatose/dying spirits and made a contract with them 3) the PCs represent strong-willed people who refuse to pass on like this and through sheer willpower cling to existence (you could put a clock on them, supernatural style, if you do this: how long before they become evil spirits full rage and desire for vengeance => they need to be revived [reclaim their bodies] 4) ask the players all to start with a level in Warlock/Cleric/Paladin & their deities/patrons refuse to let them die before they get the full use out of them [that would not necessarily be my favourite option for a variety of reasons but I have heard of campaigns like this before - I'd say it would depend on how well you know your players]

Alternatively: 

1. The players each supposedly did ~ so heinously evil that not even the hells could take them. They're cursed to wander existence as spirits, otherwise exactly the same as they were in life but they get no pleasure from it (no interaction with others, can't eat or drink, shunned by societies, whatever bad things you can think of) & they have 90 (or whatever) days to redeem themselves as spirits before they're cursed to be sealed in a solitary demiplane for all eternity with not even each other for company, but if they do really well, they will be restored to life.
2. An evil entity is wrenching people from their bodies and forcing them to fight in spirit form in some crazy arena (and the stronger ones are sent out on missions at its behest). The winners will be restored to their original bodies, assuming you can trust this thing to keep its word). 

My general advice is if you want to do the spirit thing, have at it, but don't complicate it beyond their spirits. Otherwise they run like ordinary PCs, the plot hook rather than a mechanic.

These are just my own thoughts on how I might run this kind of campaign. Hope it helps and good luck with the campaign.