I would love to see an unmanaged implementation of WPF, with the managed version transformed into a wrapper around this new foundation (similar to WinForms/Win32). This would not only potentially solve the perf problems, but also open up WPF to managed and native developers alike. In addition, they could then also potentially use WPF for core applications like explorer.
In fact I believe the work has already been started with Silverlight's implementation?
There is an unmanaged implementation, well sort of. On Vista the visual tree created by WPF gets send to milcore.dll (unmanaged) which composites it with the rest of the desktop. It's why, for example, on Vista you get smooth resizing of WPF apps when you use the Windows magnifier.
Milcore is also what powers stuff like Flip3D. Hopefully they will publically expose it in future releases.