1. The ability to play as, team up with, and summon every character you saw in AC. That is: Cloud, Tifa, Vincent, Barret, Red/Cait, Yuffie, Cid, and even Kadaj/Sephiroth. Maybe even playing as Rufus and the Turks . . . but probably no. Playing as Kadaj/Sephiroth would change the ending like in Star Wars the Revenge of the Sith on PS2.
2. Kingdom Hearts 2-styled battle system with Cinematic "Triangle Reaction" commands would make all of those memorable scenes come to life in your own hands. You'd be free to execute them at your leisure and they'd provide only a defensible advantage. This means that you could basically accomplish the same amount of damage to a foe simply by slashing and blasting them with magic.
3. The motorcycle battles would be much more exciting if based on AC. All the spinning out, wheelies, and all those swords in the Fenrir.
4. Dude. Nanaki only got one stinkin' line in the whole flick. When you thought he might talk aboard the Shera, Cait shut'em up by slapping him on the head.
5. I'd personally really like that tour of the Shera that Cid offered and we never got.
6. Midgar is only visited during Cloud's duel with Kadaj and Sephiroth. Edge is the primary city. I'd really like to see more of it.
7. We'll be seeing enough of Vincent in DoC, so we don't need a lot of hands-on with him. However, I'd certainly like to get my hands on Tifa . . .

I'm currently working on a script for making FF7 into a movie. It'll never be made (legally), but that's okay because it isn't difficult. You just take the basic script and trim out the excess.
I'm also working on a script involving the kids.