I suspect Apple is working on a truly upgradeable Pro tower. What that tower looks like and how it is upgraded are still in question. The options seem to be:
1) A standard tower stuffed with drive bays, RAM sockets, and PCI slots, much like the Cheese-grater Mac Pro/Power Mac G5
2) A modular, stackable, component-based "tower" for which you buy "modules" that simply stack one atop the other without regard to order. You buy a brain, and then add or remove modules as your needs change.
I like option 2 the best, but think it will prove to be too expensive, and its success will hinge entirely on Apple's willingness to license the connection tech cheaply enough so that 3rd parties can make compelling modules. If it can be done so that a decently-specked system can be had for $3000 to $5000, it will be successful. But these $10,000 systems are never going to be popular. Even the iMac Pro is now way over priced.
Boise State! … Boise State! … Boise State!