you can make cylinder PC case that asthetically does not rip off the Mac Pro to the extend the Dune does.
Though you did ask how much different can a trash can look. Here is a quick google jobbie, cylinder PC, original design. I'd argue this is very different to the Mac Pro ripoff that dune has put together.
The real innovation in the Mac Pro is $$$$$$ apple has the funds to create customs Components that fit into it, for instance good luck getting 2xfull GPUs in a PC version.
Soooo....given the PC components you can buy these days, a cylinder case is kinda a waste of time due to the design/space restrictions. If your want a mini factor , get a cube. Cannot see any company making components that will fit cylinder cases anytime soon. A shuttle PC cube can take 2xgpu.