It'd be a "no mastermind, things just fall through the cracks of the multiverse" style jamjar except the game takes place still actively in those cracks because there's an accumulation of Stuff that's katamari'ing in there

Maybe sometimes like. Entire buildings. For setting expansion. But mostly random junk and people who don't, like... have a concrete connection to canon worlds, I guess? OCs are good, AUs are good, nobody's straight from a recognizable canon.

I think one of the initial spaces they have access to is a research platform from some universe that knows this happens but doesn't understand how or why. There's like one or two NPC scientists who are just as stranded as the PCs but do at least have some kind of communications with a base back in their world that's working on getting them back.