I don't think Apple is very likely to use the Core i9 tech (X299 platform). Certainly not in the Mac Pro, but probably not in the iMac line either. They didn't build anything based on X99, which is the predecessor to X299. But maybe more importantly, I can't see where it would fit in Apple's product line. Technically they could drop it into iMac, and have a high end "consumer" iMac with non-ECC memory, and then the iMac PRO with workstation grade components. But I don't see it happening, mostly because it's such a niche market and it doesn't really add anything meaningful for Apple to have multiple similar products in that space.
I would imagine the Mac Pro would be based on something like dual Xeon and dual GPU.