I can certainly understand using music, weapons, spells, and names from the previous Final Fantasy titles but I don't think the inclusion of Antagonists from the other incarnations would work well. Square have always kept the games as worlds seperate to one another. Plenty of aspects carry through each (There's always a Cid, Biggs and Wedge occur in multiple games) but they've been careful to make the character unique to each "universe". Trying to pluck Kefka, Sephiroth, Ultimecia, Kuja, etc out of their respective worlds and just drop them into Eorzea would be detrimental IMHO.