Why re-invent the wheel? Just crank up the old cheese cutter Mac Pro chassis factory. Enlist Gigabyte or other 3rd party Taiwan motherboard company to make a motherboard per Apple's specification. Put in a single or pair of Kabylake Xeon CPU's, DDR4, buyer's choice of top-of-the-line & standard PCIe (either: consumer class or workstation class) replaceable nVidia or AMD GPU, 2x Thunderbolt 3 ports. Use NVMe SSD's for storage, with options for a 4,6,8 Tb. etc. spinning hard drive.
Shouldn't take until 2018 to ship one out the U.S. based Mac Pro factory door.
I'd also bet that Dell or HP would be glad to custom produce a similar-specced interim MacPro, until such time as Apple's new modular design is ready for sale. Should be ready in about a month or two. The peculiar Mac-specific motherboard firmware being the only minor obstacle in the way.
But!!!
The whole reason for the trash can was to give the idea of user upgrades the big middle finger.
Why?
Apple could have designed, even in that space, a machine that could be upgraded, expanded, had its life extended. But, they didn't. They went all custom, all 'small', all 'closed'. I remember seeing the mockups of what *could have been*. Small 'cheese grater' chassis, creative designs that all had expandability, upgradability, in them. PCIe slots, tons of USB and ESATA ports... Instead they gave us a trash can with no slots, and very limited upgrade potential.
Don't get me wrong, I was ready to plunk down cash to upgrade, but when the first reports came out about the thing being a closed box (tube) I was crestfallen. To pour that much cash into a closed box, and have to spend even more to make it usable is a lot to ask for someone that doesn't have near six figures to spend on something closed like that.
It was a slap in the face of the many who came to revere the old Mac Pro. I couldn't see it lasting, and apparently it died on the vine two years ago, and Apple is just finally admitting it.
But admitting it is different than OWNING it. Modularity sounds interesting, but given what they did to the Mac Pro already, I wouldn't hold out hope for the pendulum to have swung back to the openness of the old Mac Pro.
Apple seems to not trust their customers, and the world. Making box after box a closed and locked box sounds like a mental disease. Paranoia? Megalomania? Egotism? There is a cognitive disassociation somewhere...
I am going to need to replace my ageing Mac Pro at some point. The Bluetooth keeps dropping, and I'm just waiting for the other NIC to die. It's full of 1 terabyte drives. The thought of having to buy an array box to move to the trash can (if I could afford it) is out of the question, and an iMac would mean the same machinations. I guess I'll have to look on ebay for a faster cheese grater and hope it's not ragged out and on its last legs too. Or I go back, with my head hung low, to Windows... I got burned on my IIcx, I got burned recently on a MacBook Pro. I just don't want to get burned again...