Sephiroth actually. Mainly because we don't learn much about his backstory - yeah we know how he was created, but what about all his feats that lead to him becoming the very best SOLDIER that no one ever was? I'd like to see more of that. Plus they could build up his development a bit more, his face-heel turn goes by so fast you'd miss it if you blinked.

Zack - I know, I know, Crisis Core was a thing. Not only do I wish to forget that piece of crap ever happened, it didn't do much to expand on his character than the original FF7 did in the first place. We never learn about his life before SOLDIER or why he wanted to join it in the first place besides wanting to be a hero, and that's literally it. Surely even Square can properly flesh him out some more, and some character development wouldn't hurt.

I would also like to see the Turks and Hojo expanded on too, though it's not really necessary.