[quote=@AndyC]
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?'
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I would say be cautious. That stuff vaguely sounds fine, but I'd also say keep it vague and don't make it a huge thing that impacts bigger mythologies. As I said in the first post, consult the player characters. Maybe try and limit the crossover stuff in the backstories themselves. My Batman has no overlap, honestly, just because it felt too cheap to have Bruce have trained in Wakanda or whatever.