Let's see... I know they can't remove Seymour, him being such an important character, but maybe they could scale back the obnoxious difficulty a bit? Some of those fights are nearly impossible if you don't already know what to do.
Also, why not have the standard shared XP thing? It must be weird as hell for the monsters when I'm playing musical chairs with my team just to make sure they all get stronger in every single fight.
The arena's boss monsters are absolute bullshit, no matter how strong my party was, that inconcievable old man (Can't hunt monsters but he can contain those fucking things?) will always have something that one-shots whoever it attacks.
The sphere thing is kindof annoying, do we really need strength, magic, speed, and ability spheres? Early on, I ended up holding half my party back until I found enough of the right speheres to advance them without gimping them at all. EDIT: To be clear, I mean just have a general 'upgrade' sphere for strength/magic/speed/ability, it's a real ***** having to go find the right monsters to kill if you haven't grinded enough.
Half the celestial weapons are an agonizing chore to unlock, but I guess that's half-justified for the best gear you can get, right?
EDIT:
That you basically -need- to grind to get through by more than the skin of your teeth, even some random monsters might wipe your team. And why the fuck, when 3/7 members are down, can't the living ones replace them? We confirm by having them switch in and out that they are ready and able, but when one falls, it's "nah, they're just lying down a bit, they've got this!" It'd have certainly helped when I learned stealing from the Magic Urn enemy results in instant death for the entire team.
Getting enough of the items needed to customize gear is arduous and irritating, especially when/if you learn the item you're looking for can be attained in bulk, once, from that fucking old man at the arena in exchange for catching some near-harmless-at-that-point monsters from earlier areas.
And finally, they could quit cheating in Blitzball so damn much. I thought things were unfair in the forced match because that was the great champion team, but no, it seems the other team is always higher-leveled than you and always has ridiculous stats, maybe there's some anti-bullshit trick I missed like Auron attacking a flying enemy, but thankfully it's optional anyway and you don't have to win the one that isn't.
(Guess who's been replaying it recently!)