please don't mess with the chasis external design other than IO placements in the back and a few tweaks.
Having gone through a pair of G5's and current an active user '09 Mac Pro; I hope I am not the only one who loves the chassis.
As old as it is, don't mess with it. Over-all I still believe it is be best looking desktop/workstation chassis --- my opinion.
I know its selfish of me to want them to keep the design, but please please please keep it!
Just update the mainboard so it can take PCIe 3.0, USB 3.0, Thunderport, and latest Intel Xeons and add SSD and/or RAM disk caching. Partner with nvidia and AMD to produce GPUs for it, make the license fees lower so they can afford to sell GPUs for Mac Pro at the same price/cost as their PC counterparts.
Heck, convince them to release GPU that would work on BOTH mac and PC and delegate driver development to them! Give them an incentive to do that, and buyers like me will flock to your mac pro.
You'll probably get more game developers support too.