Mac Pro 2020 upgradability - DDR5, ARM, Neural Engine?
The short answer is no, the Mac Pro isn’t upgradeable to any if those technologies.


DDR5 RAM, is a low power, but double bandwidth compared to DDR4.  Its a standard that hasn’t been released yet, and must be supported by the CPU.  in order to support this, you would need to redesign the logic board to accept the new requirements and since it’s a non-finalized standard, nobody has adopted the tech yet.
ARM vs Intel.  This is two completely different CPU architectures.  To use an automotive analogy, this is like asking if your car with a big block V8 can use an electric motor from a Tesla.  Unless you remove all the drive train and what supports it so you can put in this new tech, it’s not upgradeable.  The same holds true for Intel to ARM; you’d have to replace the logic board (at minimum) to support it and that’s not “an upgrade”
Apple Neural Engine is a technology that comes on their ARM SoC (System on Chip), it’s not application software that you can just install and suddenly have.  This is their CPU, and to support it, you’d need a whole new architecture.  See the previous point.


However, I think you said it best in your question:


  They are all technologies that could potentially be available for Mac products within a year or two.


They are potentially available which means the standards by which they inter-operate with other technologies haven’t been finalized and thusly, not implemented.