Yup, mixing and matching will be fun

I like the 'mother' mirror idea! The characters could clash with each other over what they want to do and stuff, and it would add in the possibility of NPC factions working against/with the characters and the like

For me it's basically magic and no real tech as the key elements, then everything else can be there or not. Dragons, princesses, kingdoms and the like are definitely in my high fantasy picture though

Androids and mechanical body parts (possible cyborgs) would certainly be interesting. There's all sort of features you can explore with mechanical body parts - extra added features and such. A thought that just crossed my mind - weapons between worlds. For example if someone had a gun in the modern world, would it still work in the fantasy world? Those would be things we'd need to look at

So the same sort of starting template for female and male, adjusted to the worlds they're in? That could certainly be interesting, and they would all obviously develop into completely different people. And loads of fun stuff to explore xD If we went with a clone situation I think it would be funner to go with vastly different personalities and backgrounds - almost like they're long lost twins, but not. There's a lot to look into there

An OOC would probably be good! There's a lot to discuss