I don't see Ultima as the converse of Holy. Maybe Flare or something more from the darker side... But your concept is interesting if magic were cast simultaneously. Fire and Blizz would cancel, Thunder and Quake, Aero and Water. Higher spells like Flare, Ultima and Holy wouldn't be crossoutable...

New magic? Not much new stuff, but brining the Dia/Harm spell back in replacement of cure hurting undead is cool. Its old school, I like that. Blink was cool, but gives unfair advantage in those games. Toad is cool, or Piggy or Moogle (a la Secret of Mana). Imp acctually had a usage in FFVI and I'd like a return of that. Mini is ok too, but it didn't have a huge effect in VII.

I think more original summons would be cool. Sure you have ifrit, Shiva and Ramuh. But a higher powered elememtal summon would be nifty. Thor would be a heightened Ramuh, Cerberus would be a heightened Ifrit. I don't have a stronger Shiva, but square has a whole team of researchers, they can find one. Anyhow, I think originality of summons is sweet. Valefor was a great summon. Quetzecoatl was cool also. KOTR, cool but too long (same with Eden). Some more 1 game unique summons is something I look forward to when playing an FF for the first time. Tritoch = Sweet!