It'd be pretty cool to get the Turks on your team. Dyne would be good as well.

Rufus would be pretty cool. I mean he'd have a Shotgun and maybe some weak magic.

Bugenhagen would be hilarious. So would Don Corneo. It'd be funny if you had him in your party against Jenova and he started flirting with her. Then She left with him and they made babies.

It'd be good if Jessie, Biggs and Wedge hadn't died and one of them joined you. Or all of them.