I agree with limiting or changing some of the special skills like Blitz, etc.

I would have more statistical variety with the characters in terms of HP, Vigor, etc. Most of all, though, I would have characters have a Skill Point system, in which they can choose to "buy" spells from lists when they level up. Espers still teach summons, but will mostly affect stats as you level up with them. Upon getting Espers, spells as well as other things (like HP +10% or Cover, as well as "Dual-Wield" or "Use Maces") will replace Relics and the typical magic system. Characters like Sabin and Cyan would be mostly limited to combat-based skills, while Terra and Celes will get black and white magic, respectively along with their own spells. Each character has a more defined class, but more versatility within it.