Remake Final Fantasy:Mystic Quest. However, not a 3D remake, but redo the gameplay. Have the ability to roam free on the world map instead of being linear, have side to side battles like in FF I - VI, have a 4-5 person party instead of a 2 person party(this would require to add more story script so characters can stay with you instead leaving at some point, and perhaps they can get more dialogue as well), increase the difficulty(however, not to the point that it will frustrate the players), add weapon & armor shops, chocobos, side quests, more spells, and certain character exclusive abilities(like Benjamin can have Cover, Phoebe can have Aim, etc.). I would even make the game part of the numbered series by renaming it into "Final Fantasy 0" and make it available to online services like Wiiware or the Playstation Network.

Btw, I have nothing against Mystic Quest and I love playing it very much. This idea of mine is more for the people who does hate it.