With a advent of Fusion Drives, I was certain the next Mac Pro would come like this. Pros (whatever this means these days) needs fast access, but large amounts of storage. HFS+ makes this a tricky sell, but with APFS, Fusion Drives make more sense.
It'd be great if the next Mac Pro came with an NVMe M.2 SSD (SM961 anyone?) in a fusion drive setup with a large HDD to back it up. Perhaps tweak it to that you could have two spinning drives? M.2 would be especially welcome, not this proprietary nonsense.
Use standard GPUs from AMD and Nvidia; some people want OpenCL, some will want CUDA. They could sell their own upgrades later if they wanted to? If they're serious about gaming and VR, this could be the machine to do that?
Xeons and ECC I don't see going anywhere, and they've proved they can vastly shrink the logic board down, so it doesn't have to be a massive tower like before either. There's always scope for making them i7s, but then that cuts into the iMac margins of course
DVD/BluRay drives are highly unlikely, but no one would be too surprised by that by now.
Thunderbolt would be a basic requirement, hell the first shown Thunderbolt (then named Light Peak) computer was a prototype Mac Pro reported on MR a number of years ago.
Apple have a lot of directions they could go in. The current Mac Pro was and is certainly impressive, but I just think too many compromises were made to impress people with how small it is.