I'm not so sure if the Mac Pro is quite dead yet as it really depends on what we see at WWDC. It might be Apple's position that the M3 Ultra and TB5 will be enough for Pros (unfortunate if true). Since it does not look like the M4 generation will have an ultra chip, it could mean that the next iteration of the Ultra would be M5. If rumors/speculation of the next Ultra being a monolithic design are true, it could mean that an M5 Extreme could be introduced. This could serve to further differentiate the Mac Pro from the Mac Studio especially if the studio remains in its M4 Max/M3 Ultra configuration. At the very least I expect to see a Mac Pro with M3 Ultra with the same specs as the Studio. I think there is the remote possibility of added compute cards and/or some clever way of expanding memory beyond the 512 GB.
I'm just spitballing here, but I think the easiest way to do the memory expansion would be to have a separate bank of up-gradable ram that initializes as a RAM Disk upon start-up if the system detects memory modules. The OS and applications load into unified memory as they currently do now, and only that which the user places into the RAM disk resides there. While not as fast as unified memory, I would think that this could still be way faster than accessing the SSD and could have much larger capacity than unified memory existing as a middle ground between the two. Two use cases I can think of this right off the bat would be loading large sample libraries in Logic and certain AI tasks that require large amounts of VRAM. If the user doesn't need this capacity and say just needs the PCIe expansion, then the system works like it currently does.