I'm on favour of the idea of some transuniversal group/organization pulling several charaters from different universes for some greater purpose. The problem here is that I can't decide if they should be aware of other universes' existence previously or if the only one aware before is said transuniversal organization. Maybe some universes do know of others' existence. I have a story on which a mistake opens up a series of portals between parallel Earths and, as many of them are empty, colonization starts.