I wonder...
...if a mini version of the original IBM PC, a basic system board with expansion slots might be something to look forward too? Rather than ISA slots, I'm thinking vertically mounted M.2/PCIe slots running lengthways so would accommodate external ports on the end of the expansion card through the back of the case. Of course, not quite as basic a system board as the original PC, since so much more is already in the onboard chipset. Whether an eco system of 3rd party expansion cards could grow may be the real sticking point. It would probably depend on whether the specs were open enough and no silly patents on board shapes, sizes etc.
It won't happen, of course. The way to make huge gobs of money is to make everyone buy the special custom box they need, not allow them to spec and expand as required to make their own custom box with just the bits they need.