Just make it a cool looking square and shove tons of hardware in it. The whole circle thing is such a bad design.
Instead in 2020 we'll probably get something like the desktop computer version of the RED camera system, and priced to match. Something that Apple's very very VIP consultants can use and afford but which doesn't meet anyone else's needs or budgets.
Though the big aluminum desktops they made years ago were probably a high point for balance of beautiful and functional.
This. I think existing and potential future Mac Pro users would be perfectly happy with a cheese gater that has 2018 innards (ok, they could make it Space Gray).
When it comes to Pro hardware, I think that Apple has lost the balance between aesthetics and functionality. The trash can Mac Pro is a good example of this - it's beautiful, overheating all the time and not extensible. So is the new MacBook lineup, they are beautiful. But really? 1 USB-C port on the MacBook 12" and 2 USB-C ports on the MacBook Pro Escape? I don't know of any MacBook 12"/Pro user who doesn't dislike the port selection.
The NeXT Cube had three NuBus expansion slots that accepted boards something like 11"x11". (There were actually four connectors on the backplane; one was used for the NeXT motherboard.)
Granted, there weren't many boards produced for use in the Cube. There was NeXT's own i860-based color video card, the NeXTDimension. I think there was a third-party card with multiple i860s, and there was a 3rd party card with multiple Motorola 56001 DSPs.