This is a pretty simple premise for a jam. Find a TTRPG that you think is cool but doesn't have enough extra adventures/expansions/whatever, and make an adventure/expansion/whatever for it. 

It doesn't have to be polished. It doesn't have to be balanced. It just has to exist. I know I'd be thrilled if someone did this for one of my games,  so I'm sure the designer of whatever game you're thinking of would feel the same.

Rules I Wont Be Enforcing:

Make It Indie: Look, I'm encouraging you to pick a relatively unsupported game, but to enforce this I'd have to define "Indie" and I can't be bothered with that.

Make Something New: It would be in the spirit of the jam to make something brand new. I'm probably not going to check though, so go mad.

Make It Scrappy: I want everyone to feel they can enter regardless of design experience. That said, I'm not gonna tell you your game looks too nice to enter.

Rules I Will Be Enforcing:

Check The Licence: Before you write your cool thing, check your allowed to. Mist games will have something on one of the earlier pages, but some might have a license elsewhere. Just Google it.

Don't Be A Prick: You know who you are. Don’t use this space to push your hateful views.

Don't Use AI: Like, at all. Not for writing. Not for art. Not for any thing. And dont come at me with a "Ooh, technically this isn't AI" either. You know what I mean by this rule.

That about covers everything. Also this was inspired by a post from Justin Vandermeer on the Plus One EXP sever, so go check them out for more cool, indie stuff.