While there may be a few manufacturers that try to mimic the design, the target market for those that will use them is small. I don't expect to see many.
What will be popular with this new Mac Pro are rack mounted solutions that hold multiple devices.
A rack mounted housing for your existing Mac Pro's HDD's that connects through TB would be a hit.
Any TB based docks/extensions that can be made into rack mounted accessories will be popular.
The new machine is targeted at media creation pros. We've got lots of racks to throw stuff on and not a lot of desktop space.