People have already pointed out most major ones, so I'll try to think up some original ideas.

FFI - Add more of a turn order bonus for faster characters

FFII - Have spells change effects as they level up

FFIII - More consistent itemization to make all the classes more or less viable the entire time.

FFV - Make it so job stacking is a less viable tactic. This could be done through itemization like in FFIII (you only have the gear to equip one person as that class at a time most of the time.) or through generally needing a more balanced party.

FFVI - Same things most people have said, balance out special abilities, make desperation attacks more relevant, buff espers.

FFVII - Buff green materia, next to enemy skill and other command materia it is seriously weak. Summons need a buff as well.

FFIX - Speed up the pace of the battle animations, so I don't always have a large que of actions waiting to happen

FFX - Make Overdrives less easy to hold on to. I always kept them full, and when I came to a boss encounter I just blew all my characters and Aeon's Overdrives and it was usually over.

FFXII - I would need pages and pages....