[@joiedxvivre]
Okay but, why does it have to be some realm? Sounds like we'd just be winging it with apparently nothing to make the crossover aspect relevant or central, don't you think? By that point, to fully enforce the idea of requiring training it'd feel more sensical to just have, for instance, the chaotic state of the world weakening the fabric of reality to the point that magic becomes active once more and technology advances at a breakneck pace. That way you can have a wide variety of characters on a consistent setting that actually require training and development and adapting to a changed world without having them be existing characters for some reason.

Aside from the multiversal aspect feeling a fair bit tacked on, it feels like there's a disconnect between the general attractiveness of crossovers (established characters that can already do things people/players know and like) and actively taking away their memories and skills. Every character building aspect you brought up kinda hinges upon the characters' stories that you're taking away, so blanking them runs rather contrary to that for the sake of a development that doesn't even need to happen that way. Development can well just be adapting to an entirely different world.