If people want to get rid of "everyone can learn all spells", I think a good example to follow would be FF XII International's Zodiac Job system. Pick one class that can only learn a certain amount of skills and spells, and you're stuck with it for the rest of the game. That'll keep the magic pools unique. How many characters did VI have again? 14? You could asign each of them a zodiac job (with two that will end up re-using an already used job by another character) and have a fully unique cast with a wide variety of magic lists.

Or something like that.