-Full BluRay (Menu) Support
-Full 3D Support (Frame Packing, SBS, Over-Under)
I think these are unlikely to happen without Media Center becoming a licensed Blu-ray player - which would mean restrictions on what the program is allowed to do with audio, and may even prevent things like madVR from being used.
As far as I know, there are no projects out there for Blu-ray menus or 3D support which are anywhere near a state that they would be suitable for MC integration.
And many (most?) people running software like Media Center actually
prefer not having to deal with Blu-ray menus, with the software automatically playing the film as soon as you start playback, and not having to configure audio or subtitles.
Menu support also means things like being forced to sit through 10 minutes of trailers before you can even get to them.
Madshi has mentioned that he would like to add 3D support to madVR, so if that ever happens, we may see 3D supported that way, but that could be a long time away. I think it's a low priority for him.
There are only a very small number of real 3D titles out there, and the industry considers 3D to be a failed venture, with focus shifting towards 4K now.