Mojang are in fact working on the long-awaited 'Adventure Mode', and though it's really intended for map makers, with a bit of tweaking it could become an interesting RPG-esque game on it's own. 1.3 is also a hell of a lot more mod-friendly than previous versions, and this is something that Jeb personally has stated that he wants to focus on in future releases.
In terms of actual features though, I think Minecraft itself is as 'complete' as it needs to be, barring bug fixes and patches of course.
EDIT: That said, I would like to see a collaboration between Mojang, Formivore (CARuins, Walled City Generator etc) and Chocolatin (Better Dungeons) in order to make some of those features vanilla - I love love love my ruined tower base I set up on my SSP world :3