1) Status integration with facebook and twitter
Very important, the members use more often twitter and facebook instead of the forum notifications...a integration (like in ipb) should be great).
2) Recent Status Updates Page and Widget
There are already a Widget for this, but is incomplete and a feature like that should be in the core. I like the "Recent status updates" and the Widget present in IPB, but i think XF can do better!
3) User Tagging
Very useful, there are already an addon, ma i think should be in the core (cit.).
4) Social Gallery
I don't have ideas about that...have you ideas about that?
5) You May know...
Features and and a widget to increase the user interaction/socialization.
What social features in your opinion Xenforo need?