There isn't going to be a "next Mac Pro". That is what I believe. But I certainly hope that I am wrong.
That's probably true because it's about marketing.
The Power Mac was justly named because it was a power users Mac vs the iMac which Apple marketed to consumers and education.
With the switch to Intel processors the Mac tower then became the Mac Pro to differentiate it from the now much more powerful upgraded iMacs.
Now that the iMacs along with the Mac Mini can be a solid solution for many professionals Apple needs to again change the name of their most powerful and expandable desktop. That name will really depend upon what this "something really great" is.