FFI
Like everyone says, make the magic suck less and use MP like the later games instead of spells-per-level.
Decent upgrade for the BB

FFVI
The "missing year" scenarios would have been a nice addition. Every character would have had their own solo adventure prior to Celes tracking them down.

Yes, it the WOR was way too non-linear. I think this was in part because of how soon we got the second airship.

Yep, the esper summon did seem pretty pointless. There was also too much duplication of the less useful spells.

It would have been nice for there to have a been a "best weapon" to get for each person like there was in Chrono Trigger.

Some characters were too strong and others were pretty pointless to have.

More split-up type scenarios where you have three or more groups doing seperate adventures all over the world at once. This would justify having 14 characters.

Honestly though, with that many people it seems pretty hard to come up with a fighting style and abilities that are both useful and unique to each person.

Cyan's sword techs are useless as hell. By the time he charges up, the battle is over. No good in battles with a time limit. The only time it was worthwhile was when the enemy was doing an attack with a big complex effect that took a long time to do.

FFVII
Tifa wasn't naked.

(just kidding)