The biggest question is: does Apple want to bring back the Xserver? If yes, there are several ways. If not, no good proposal will sway them :) But one thing was interesting during the AS announcement at WWDC. Federighi stressed how good a hypervisor macOS is. That was probably first of all a comment about virtualization on the new Macs, but could also allow to install any operation system as guest on a cloud server. Many cloud services are running inside VMs.

The only reason for that statement is to silence those who would raise a stink over the loss of Bootcamp.

"See, you don't need native Linux on the machine, just use our hypervisor & Docker/VMs"

Yes, that was the context of the statement. But it also would give an answer how customers would use an AS Xserve.