What's the acceptable extent when it comes to crossover stuff?  F'rinstance, if I were to incorporate the Inhumans into the backstory of Krypton, or say that Asgard was bottled up by Brainiac or something like that, which might preclude other people's ability to use a character or a major mythos in a way they want.  Essentially, where do we draw the line between 'mixing it up' and 'stepping on toes?'