Ooh, the mirrors idea is interesting! Mirrors are always a trusty way to construe parallel/different universes. Modern, fantasy, and sci-fi/cyberpunk just about covers everything, so I'm okay with those three worlds, too! :D 

So what were the possible roles you were thinking of? I can think of one where would we essentially have the same female and male character, but each of us will be playing different versions of them. That could be interesting to explore (lots of parallels and foils and all that fun stuff), but we might also be slightly more limited with relationship dynamics (though not by a lot).

EDIT: Haha, we posted at the same time!