Dungeons and Dragons' Wizard Class sounds a bit like this. Each spell level has X spells you can learn, but there are Y amount of spells. Y>X so you can never know them all.
However you can increase the amount of spells per level in D&D unlike final fantasy, could be a nice choice for the gameplay to have some way to get a few more spells.
Unlike D&D you'd want spells to be reusable right away to avoid going to sleep after each fight, manabars always seem to work fine for that.
The balance could be like pokemon, which is what i'm getting from Zul'Jin's post. Pick Fire and Gym #1 is hard, but Gym #5 is easy
Pick Water and Gym #1 is easy but Gym #5 is now hard!
I can't really think of a game that has balance right. Final Fantasy generally has overpowered auto attack, Diablo 2 has useless spells as does D&D. Pokemon had the big Psychic imbalance, WoW has warlocks...
It's so important I can't belive they miss some of the stuff that gets though, like I just started replaying FF12 and all my characters ever do is auto attack. Thing is that I can cast that fire spell and do 85% damage to that flan, but I'll spent a little extra time poking it with sharp sticks and use that mana to use cure later thanks.
Another FF exampke (yawn), but FF10 got it kinda right. Each enemy was weak to a certain type of attack, so you had to train all the characters and match themto the enemies like rock/paper/scizzors. If you used the wrong person to attack you either missed or did very low damage.
Unfortunatly I managed to break that system because there was a little freedom in that the fast character could kinda kill the enemies the accurate character could if he gained a few levels, and you give him a certain kind of sword and he starts killing the enemies the powerful/magic characters usually took. Spread that over three characters and your sorted.
But it was damn fun figuring that out and playing the game designed for ~7 characters with 3. Some kinds of imbalance are great if you can think up your own strategy to 'break' the game and get an advantage. Really precise balance like Blizzard uses is a bit soul destroying when thinking out of the box is nerfed.