I'm anticipating there are few things they could do with this. One is that Quadratum could be just a prologue world, and then maybe a hub world you return to, but Sora will manage to return to his original realm early in the game and we can explore Disney worlds, but frankly, I don't see that as likely, and I think Sora will be here for most of the game.
Another possibility is that they make the Quadratum realm itself an entire universe full of other "worlds" to explore, maybe based on FF games, which would be super-tight (and give me a good excuse to go back to playing FF again, it's been so many years). In that case, I imagine all the Disney realm stuff would just get told in cutscenes.
Or, another possibility (and this is the one I'm hoping for) is that we alternate back and forth between playing as Sora in Quadratum and playing as other characters in the Realm of Light, and they all get together at the end.