Hmm, the discrete GPU would be interesting, but I don't think would go back to that. The SoC model has been working out great for Apple, I can't see them seceding control to get a discrete 3rd-party GPU working.
Nobody said the discrete GPU would be 3rd party. I suspect Apple will release their own discrete GPU before long. SoC works well for the long end, but I can't see it being worth it at the Mac Pro level.