If anyone here is familiar with the star ocean series, then you might know what a hard mode can really do for a game.
The first time you play, everything is foreign, you make a ton of mistakes, and you eventually learn how to work the mechanics of the game. Then once you do EVERYTHING IN THE GAME IS RIDICULOUSLY EASY. At that point what you want is something that will actually require you to apply your new mastery of the game - where the game will absolutely massacre you if you do not use every trick in the book.
Doing this requires virtually no additional content (although it COULD be added) and instead just some formula/resistence tweaking or whatever other kind of tweaking they can think of to make the game tougher.
Every single Final Fantasy game I have played suffered from not having this. Every one had really interesting mechanics that could be toyed with to make your character more powerful which at first would captivate you and get you power leveling until finally you realized - There wasn't a single enemy in the game that a person without a clue in the world couldn't beat.