I have not looked to see if this has been suggested before but here goes.
It would be nice to have the windows desktop captured into a texture so it can be displayed on a quad or similar.
It would be really nice if it could be done under a DX9 fullscreen (on one desktop) and also with the ability to send mouse and keyboard events back to windows.
Basically windows on a quad. But without the vvvv renderer visible. Maybe normal vvvv with selectable visibility.
I have seen windows desktop transformed in 3D so it should be possible.
EDIT
VNC support is kind of related. (would be cool)
Maybe the DFMirage driver could be helpful.
http://www.demoforge.com/dfmirage.htm