Yeah I'm pretty sure there are scenarios where you need the horsepower of a Mac Pro and small size. Say for example oh I don't know, editing movies and need to travel with equipment. Putting a MacPro in a pelican flight case is very annoying. You need a flight case which weighs 50 POUNDS for just the case itself which makes the whole arrangement just about the size of a dishwasher.
30"x30"x16"
People think you're carrying military ordenance with something that big and it's a lot of trouble to lug around.
Apple could easily make a system that is much smaller, there is so much wasted space on the Mac Pro. With computational fluid dynamics they could easily make the air flow work flawlessly too so don't tell me that would be an issue.
A modular design would also be awesome with seperate PCI Express, CD/DVD/Blu Ray Drives and seperate HDD bays.
Even better make it a 1u rack sized, and 4u with all the expansion bays installed.
Image
Imagine this, except as a Mac Pro.