I think they will need to support DDR5 ECC memory modules or there is no point in producing a new Mac Pro.
If they throw away the performance advantages of Apple Silicon, like on-package unified RAM shared directly between CPU and GPU and GPUs optimised for MacOS/Metal, then there might not be any point transitioning the Mac Pro to Apple Silicon. Just keep supporting the 2019, which isn't really obsolete yet.
Apple really needs to make a server class processor for the Mac Pro
Mac Pro isn't designed as a server, it is a high-end
workstation - designed for interactive use by a single "power user", and the Xeon-W isn't a server-class CPU, the 'W' stands for "workstation". Sure, you can call any computer a "server" if it's sharing files or running a web server, but computers
designed as servers have quite different design priorities. The only time Apple offered server-class hardware was when they made the XServe. Heyday of the XServe was when the PPC chip was still cool, before Linux had so much industry acceptance, and MS Windows Server was knobbled by huge per-user licensing fees. Today, in server applications where the MacOS UI isn't significant, there's no advantage to MacOS on a server and no real reason for Apple to have skin in the server game.
Amazon and others already have server-class ARM chips that will out-server Apple Silicon, and run Linux which has supported ARM since the 1990s & who's developers have a culture of hardware independence. Apple would be playing catch-up & designing a chip for a tiny niche of customers who want a MacOS server.
Then who’s going to directly compete with 4090?
Not Apple. Unless unless they do
two U turns (a W turn?) on (a) supporting non-Apple GPUs on Apple Silicon and (b) burying the hatchet and having anything to do with NVIDA.
Your 4090 will work very nicely in a bog-standard Xeon-W or AMD Threadripper tower, and if you're doing the sort of work that needs one then it won't run significantly faster or cooler if it is plugged into Apple Silicon or x86.
For some tasks, maybe a cluster of Mx Ultras would be more power-efficient than a single 4090 - for which a 1U rackable Mac Studio or Mx Ultra "compute unit" on a MPX-like card (maybe even plugging into a 2019 Mac Pro) might do the trick.
Reality check here - apart from whatever advantage you may or may not get from the T2 chip running the SSD - the 2019 Mac Pro is just a particularly fancy-looking Xeon W tower "blessed" to run MacOS and a neat scheme for routing extra power and Thunderbolt to "MPX" cards which are otherwise the same hardware that you'd plug into a PC. They're of interest to a small but deep-pocketed niche of users with high-end pro workflows expensively tied to MacOS, a niche that is probably going to shrink. Apple
could develop a custom ASi chip for that niche but without high-volume sales from MBPs etc. it would cost a fortune - and if it didn't provide a
seamless transition for those complex, legacy workloads, many of the potential customers would take that as a nudge to switch to Windows or Linux.
Machines like the MBP and Studio are a much better use of the
strengths of Apple Silicon - I'm not sure it makes sense trying to turn it into something it isn't.