I'd like to see some new materia. Additional story scenes. But imagine if they lost their minds and go totally over the top:
.......it's fun to imagine anyway ;_;
The party finds a time machine in a Shinra laboratory and use it to travel back in time to pre-Nibleheim. Through a series of events Sephiroth joins the party and travels back to the present with them, where they inform him of the truth of his origins, and how present Sephiroth is attempting to destroy the Planet with Jenova and Meteor. Rather than going insane, hero Sephiroth joins the party to fight against himself.
.......it's fun to imagine anyway ;_;