the main point for minor improvements in the old games is to mainly fix the weird errors and bugs that existed. int doesn't work in ff1, weapons with elemental properties don't work, if two characters target one enemy and it dies from the first attack, the second attack won't auto-lock on to a different enemy, the character will attack empty air. it took me a while to figure this out the first time i played nes ff1 on my wii.
nes ff2 has an exploitable bug with the stat leveling up mechanic. you can select an action, let the game select a target, cancel and repeat. it builds up the experience bar for weapons and spells.
i'd say tweaking the mechanics for ff2 by using quest 64's stat mechanics would help quite a bit with the game.