In my opinion they will use an SoC for the Mac Pro but in the same way that AMD do on EPYC. No separate chipset. Storage, Audio, USB and so on all within the SoC. Graphics will be handled by dedicated cards. This is all how it's done in EPYC.
The big question is will they use chiplets like AMD has on Epyc, Threadripper and Ryzen or will they use a monolithic die like Intel? - I suspect for the Mac Pro they will go with a chiplet approach.
We can only hope they go with chiplets. At the very least, they could shove a lot of cores into a Mac Pro that way for folks that do need it. But they also get the yield benefits on smaller unit counts that AMD is making good use of.
I don't think we'll see that. With work really focusing on the GPU and accelerator cards more than the CPU. Just my opinion though.
Which is why I’m baffled they didn’t mention one thing about GPUs, PCIe and TB3 (or USB4). They could have spoken to it at least, and the DTK isn’t much help, other than pointing out the I/O limits of the A12Z. I’ll be devouring the WWDC sessions this year in part because there is so much, but also because I want to see if there are any hints in the ARM coverage.
But again I don't know how Apple does things. Clearly it took them ages to do this current Mac Pro. You may remember they brought in a bunch of journalists and well-known Mac enthusiasts to their design lab about two years before the new Mac Pro was shown off and told them they were planning a new Mac Pro. And in that case they weren't even making the CPU it uses.
I fully expect the 2 year lag was mostly farting around with trying to make it perfect rather than getting it out the door on a schedule. The insane attention to detail that allows 3rd party drive cages, logic board carrying the 12V rails instead of cables, MPX slots, fan-less GPUs, etc all scream that.