I hoping for a final showdown with Sephiroth, in a secret world. The world will be like the part of FF VII where you were inside Clouds mind. Cloud and Tifa both join your party and you go face Cloud's dark past. Maybe throw in a Jenova mid-boss or 2.
After making your way through the world, you confront Sephiroth for the last time. It will be a multipart battle, with party members present the entire time. Sephiroth starts in human form, only much stronger and faster than before. After you beat him, there will be a Cloud vs. Sephiroth cut-scene before Sephiroth transforms into a Monster. After beating his final form, you claim victory and some sort of prize.
Problable won't happen, but it would be nice.
Other FF bosses appearances would be welcomed as well.