I want pretty much everything Locke's already mentioned-particularly the part about a large cast that isn't already set at the start of the game, AND some characters like Mog, Freya, or Cid from FF4-and smurphy's bit about towns. More NPC's, plz! Like, ones to care about, who actually effect the plot for more than just a couple of minutes. I'd also like to get back to a more fantastical setting, a la 9. Or maybe a steam punk setting like in 6, or even Tactics. 6 was my favorite, so...

Everything they did in 13, I want them to take a step back from, essentially. It was too linear, all of its attention was on the main party and practically none was on the world around them, and in spit of having special powers and inhuman strength, they all just felt like fairly ordinary human beings. I want some oddball characters, characters with real checkered pasts, tension in getting along due to differences in culture or backgrounds, learning to overcome those differences, etc.
Spoiler:
Yeah, Fang and Vanille came from Gran Pulse, but they didn't seem to have that different a culture. They very easily fit in with the rest of the cast. And yeah, Lightning didn't get along with anyone at first, but that was because she was a bitch.
Anyhoo...

Wuv, Yer Mom