[quote=@Roman]
What degrees of the supernatural are acceptable, and how much leeway in rewriting backstories are we granted?
[/quote]

I'd say it needs to have the same level of potency and influence as any other set of super-powers.

As for rewriting back-stories, it needs to be primarily based on official sources. The comics, movies, TV shows, cartoons, video games, etc, are all usable, as are well-known alternate universes like Ultimate Marvel, and you're welcome to mix and match elements as you like. However, rewriting to the point where they are no longer fundamentally the same character is not allowed. I'd also say pulling some obscure one-appearance multiverse variant is probably also not allowed.