So a while ago I heard that Yoshi talked about adding the Gold Saucer to FFXIV. If this is true there is a HUGE amount of potential that could be added to FFXIV making it more unique than most if not all MMORPGS out there and add a HUGE amount of content to the game.
Chocobo Breeding/Racing
Adding Chocobo Breeding and Racing to FFXIV would give players something to do other than grind crafts and dungeons. I would argue that there would even be a huge audience willing to focus mainly on just this aspect of the game. Different colors of chocobo's, stats, attributes etc. would give your chocobo a more unique feel. Being able to race your chocobo at the Golden Saucer for prizes would also add something unique to the MMORPG experience. Much like FF7 players could also place bets on the races adding even more of a reason to earn gil and keep the economy healthy. If betting gil would be an issue then make it to where you can bet a unique currency specific to the Golden Saucer.