Also, make World of Final Fantasy and their Kingdom Hearts appearances canon. Just have it so they're all in Midgard or whatever, pootling along, and then WHAM! they get sucked through a portal into chibi world and you have to play through World of Final Fantasy for ten hours before you can get back to the main game. Then, a few hours later WHOOSH! and they're in Kingdom Hearts world for no reason.
Serious answer: keep it simple, SE. A few minor inclusions from the Compilation etc. will probably be fine, but nobody seems to want huge swathes of the extended universe in the VII remake.