I'd have a "cast anytime" Blue Magic system. If it were Limit Break only, then that would drop me in the category of "a game where all the characters learn the same basic set of abilities", which can get a bit dull... Don't get me wrong, I love FF7, 8 and (admittedly) 10, but having a whole bunch of characters who use the same basic abilities and are only set apart by their Limit Breaks gets kinda dull; I know 5 had the class system which anyone could use, but you were still limited to the few abilities you equipped. "Mighty Guard" isn't much use when you're running out of HP to protect.