I'd lower the damage on spells, and then limit who could learn spells, and how they are learned.

Most characters would only be able to summon espers, and get stat boosts from having them equipped. Basically only the women would learns spells. It definitely would not be Ultima spam fest! (Which should only be able to be learned by Terra, Celes, and Relm. But I'd make it a little weaker.

Strago would be all about the Lores. Might need to rebalance some though so they aren't too powerful. Like Traveler. Maybe cap it's damage based on level.

I'd also Make it so Terra, Relm, and Celes each had different, balanced spell lists available to them naturally as they level. (As currently with Terra and Celes, except maybe a little different). Also they could learn spells faster with the appropriate esper equipped as they level (instead of getting points to learn spells). For instance with Ifrit Terra might learn Fire 2 twice as fast. She needs to have 22 levels to learn it naturally normally, in this case every level she gains with ifrit would count as 2 levels towards her learning fire 2. This also helps with stat boost abusing if you want to learn spells faster.

However some spells would be harder to learn because she isn't naturally affluent to learning it. Maybe she learns Ice spells a bit slower. Ice 2 might only be available to Terra if using Shiva for x amount of levels. not at all without using Shiva.

Naturally I'd probably align Relm with thunder magic. Since Fire and Ice are spoken for. Terra would learn cure spells faster than Celes, and Celes would learn Status spells faster. Maybe Relm would be balanced with how she learns spells. She would learn cure spells slower than Terra but faster than Celes, etc.

Make the enemies tougher.

Remove the Economizer, or make it unique. Though only 4 people would benefit from using it now.

Make spells cost more if using the Gem Box.