FCP.Co has 2 articles from 3D designer Peter Zigich on possible, though pretty out there designs for a new MacPro which Tim Cook has promised we will get this year (hopefully at WWDC).
The first is on a modular tower, and the second a smaller design using low powered AMD chips (wouldn’t that require a new OS and apps?).
Interesting ideas, though I doubt either will happen. I am hoping for something really powerful, and better would be the ability to use PC videos cards without Mac supplies firmware as long as you have drivers, and releasing the 2 GB of video ram limit, NVIDIA TESLA support would be great too, as well as many hard drives as you can stuff in there!