Give us back our RPG classes. Just give us back our healers, attackers and defenders instead of our medics, commanders and Sentinels which belong in a shooter game. If they'll do that I'm happy, if they don't do that I'll just keep on playing the Japanese version

Apart from that I can only come up with a few minor changes that would make the system of FFXIII even better.
1) More freedom in the Crystarium Grid. Like FFX or FFXII give us a huge board and let us decide which way we go.
2) More customisability of your party, in other words if we can't control them they should at least bring back some kind of gambit system.
3) An actual 'big' mini game. I don't really mind what, but as long as it is as big and 'important' as Triple Triad, Tetra Master or Blitzball I'm happy.