Final Fantasy VI - Write a story that is about the player characters, not just Terra and Kefka with a bit of Celes and other characters to make the game seem longer and more interesting then it actually is. Also change the ending into something that is actually, well, a decent ending, rather then an extremely long, drawn out escape sequence which would have been better as a playable cutscene or something. On that note, fix all the god forsaken glitches, which made some features completely pointless. Also, give some sort of clue what to do in the colloseum - how the hell are you meant to figure that crap out.
Final Fantasy XII - Fix the audio so it doesn't sound like the VA was done in an empty, sealed, warehouse on bad mics. Add in the International: Zodiac Job System features into the original game. Make the game a little less "loolololololol good luck doing any of the extras without a guide!" .
Final Fantasy XIII - More cutscenes with Rosch for development. Nabaat would need more, but its hard to figure out just where, and maybe Raines, but that would probably ruin the ambiguity in his character. Also, add more variations to the map like in the Sunleth Waterscape of Taejin's Tower so people will stop complaining about 'linearity' even though it's just as linear as IV and X. It is especially noticeable in Chapter 10 - lord that chapter was painstakingly boring, its like they didn't even try and shake things up, even with the boss battles. Better balance - pretty much everyone will use Lightning/Fang/Hope during story and Fang/Vanille/Sazh post-story. Make the Eidolons for Fang, Hope and Vanille feel much less then a last second "oh wait, need to give them all their Eidolon", possibly giving Hope his in Chapter 7.
Final Fantasy Tactics/War of the Lions - Pretty much the same problems as VI - characters are next to non existent and their plots are resolved by the time you get them in the party, and the cutscenes in the PSP version, while beautiful, were horrendously used, and all came down to:
"Delita talks. Delita pauses mid sentence. Screen goes black, music stops. Delita finishes sentence. Cutscene resumes and probably ends"
Especially considering all the potential that those cutscenes had. Plus the ending from about Alma miraculously splitting from Ultima was just :/ worthy. Not as bad XII wise regarding secrets, but there's still the infamous battle in Riovanes Castle that forced players to restart their whole game (2/3 or 3/4 into it too, iirc). Also, some classes were imbalanced, such as the Orator for it's pointlessness, Samurai for it's brokeness, etc. Speaking of Samurai, they need to fix the bugs in this game too. Nowhere near as horrendously bad as in VI, but still there.
FF: 4 Heroes of Light - Bad balance between crowns: Black Mage vs Alchemist anyone? Some crowns could also be a lot better designed, such as the Merchant crown, which while powerful is still hideous. Also, the inventory space was horrible. Even if it is going for that classic design where everything is hard, does the game really have to go as far as lack a tutorial, or being nigh impossible in some parts?
Thats it for now.