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.