The first that comes to mind for me is that there used to be some sorta shared multiverse that's gone undone and now all of the threads have kinda intertwined at one spot (I call that space Troika! because I like saying Troika!) and the PC's have some insight into how it happened. That could be, I dunno...
They took part in the Big Unwind in some way. Assistants to the architects, if you will.
They've got a Mysterious Omnipresent Thing that wants Them Specifically to fix this mess. Why? Divine Lottery called their numbers.
But this is literally just me spit-balling off the cuff. There are so many weird things you can do with Troika that work well with the character creation concept. Go pure gonzo and have it just be a giant mishmash setting; or, have the cultures that you create exist as parasitic species on interplanetary Titans that have gotten a bit tired and fell asleep on a strange new world where there happens to exist a Titan-Killing Gun; or, Troika! is the name of an esoteric art gallery where every painting/installation represents a different culture/character concept and a modern day wizard decided it would be fun if this eclectic mix of art all came alive.
Just go wild with it!