I'll do two as well.
FF2: Replace the current leveling system with a skill/stat point distribution system where you gain points after battle and can freely (but permanently) distribute them to any base stat, spell, or combat skill level. Make some of the "guest" characters be usable permanently later in the game. This would be similar to a sphere grid, but I don't intend for there to be a lot of limitations on what you can select. Given enough EXP, you should be able to max out every statistic, but because of the exponential nature of most leveling systems, maxing out everything would take an extreme amount of time, and the leveling speed should be balanced around having a couple of weapons at a good level at the end of the game, or fewer weapons leveled but enough spells leveled to be a decent mage, for example.
FF13: As said earlier, replace narrow-corridor level design with something that is a lot more dynamic and exciting. Allow us to go back to previous areas, give us more non-combat areas where you can interact with the world. Let us learn more about the world and its inhabitants through in-game conversations with people living there. Implement most battle elements that were added in FF13-2, except monster training. Add more side quests earlier in the game, and add more side quests that are more interesting than "read mission objectives, kill, return".
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