As the title suggests, none of this is final. I literally just came up with it today and I haven't set anything in stone yet.
So anyways, my idea is a story centered around side-protagonists (so no generic shopkeeps from a JRPG or anything, people with a name and impact on the story but not serving as the main point of view) from different media getting invited to a sort of... Vacation home? Vacation neighborhood? Not sure what it's called. Anyways, said characters get a letter saying something along the lines of "We feel your contributions could use appreciation!" that invites them to the place, so a bunch of these wildly different side characters from wildly different series all end up in this one place. And for the first part or more, it'd be a sitcom-ish story about this ragtag group struggling to get along well and also live a rather mundane life. However... There's the potential that, as the story progresses, those at the vacation home could discover secrets that suggest something less than wholesome at work.... Or something like that. =P
So yeah. That's about it. The collab part comes from selecting interested folks to control two(?) side characters from some series of their choosing, no original characters or fanon additions. Aside from a few chapters that require input from multiple writers (probably handled with a Google Doc), it'd mostly be an at-your-own-pace setup, with each member writing a chapter from the perspective of their chosen characters. If this actually becomes a thing I'll formalize a selection process but for now, I'll just listen to people who habe anything to say.
I'm sure the concept needs a ton of refinement, and truth be told, I'm not sure I could actually make this happen? But if things work out alright and enough people would want to see this... Well. Time to hope for the best.