Graphics would be a nice change, up grade them, so they look like real people, people with hands, not stubs. More like 8, since we're sticking to a PS1 version. Or have the characters look like they do in battle. They did good there.
And instead of killing Zack off in the past, have him be arrested instead, so we can see him later in the game. Rescue him from Shinra tower at the same time as you escape. And make him a bit more powerful than Cloud, but have it even out towards the end of the game.

Yeah.... that'd be sweet. Anyone else agree?
Allow a four party system.