Apple explained that these “Space Gray” cylinder Mac Pro models are “designed around an innovative unified thermal core” and have not only faster processors than earlier models but also dual workstation class graphics processors, faster memory, and faster PCIe 2.0 x4 flash storage. Instead of internal expansion, the system provides extensive high speed ports for external connectivity options.
The 2013 Mac Pro redesign saw Apple opting for a cylindrical shape with multiple expansion slots, CPU sockets, internal drive bays, and six Thunderbolt 2 ports. The big issue, however, was with the graphics chips. With the Mac Pro, Apple’s idea was to fit two smaller graphics chips,but it didn’t really work.
Being able to put larger single GPUs required a different system architecture and moral thermal capacity than that system was designed to accommodate. So it became fairly difficult to adjust.
Though Apple wouldn’t reveal what the 2018 Mac Pro will look like, it says that the modular concept will be adopted. If we were to speculate, this could mean that users will likely to be able to swap out their own drivers and GPUs if they want.