The auto complete UI will indeed be improved in X - at the very least it'll allow you to type while its up. Making this sort of UI was never very practical with the setup that Sublime Text uses*, but it'll be very doable with the new GUI toolkit in X.
I think I can live without the mascot though :)
* Sublime Text has a GUI toolkit oriented around GPU rendering. This is fine for the most part, but Windows takes around ~1s to create a window that supports GPU rendering. This makes any sort custom popup window, as would be required for a proper auto complete system, infeasible with this approach.