Yes, the characters should mix and match, different dynamics to explore and all.

In think with the mirrors, there could be some "mother" mirror of sorts that broke apart in all three words, thus our characters could have shards and if we wanted, NPCs could have shards or there could be a section or group dedicated to rebuilding their mirror or destroying it or using it for evil, which could totally be a possibility, depending on what we want to do.

In terms of fantasy, medieval would be in there too, we'd have to all be on the same page since some people have different ideas of what fantasy entails. For me, it pretty much means: dragons, princesses, magic spells, kingdoms, no real tech, etc.

I was thinking for the cyberpunk, it was be more high tech and deal with the possibility of perhaps (if we want) androids or at least mechanical body parts if needed for someone (ex: guy needs a hand, has a robotic hand). Or like virtual reality and possible space travel. Like fantasy, this can be as broad or as specific as we're all comfortable with and all understand.