If it is done similarly to the 'enhanced' PC version, the backdrops won't be enhanced at all (they will retain their original resolution, being rather pixelated)- making higher res assets would take way too much time and money for just an iOS game, and it seems they don't have the original, higher res renders. So the only part that would actually use the retina display would be the character models. The only way you'd see a vastly improved FF VII game is maybe if they remade the game for 3DS and eventually ported it, but its doubtful they'd remake it for the 3DS. It's most likely to be remade on a Sony platform- so the PS3/4 or Vita, and that would likely not get ported to iOS.
All the previous FF ports were old handheld remakes. Those remakes didn't get ported until 4+ years after they originally released.