The base of the board would probably only consist of interconnects. The modules would have the necessary hardware for the compatible components. There would immediately have to be a standard formed for all interconnects and they would have to be strict.
If done right, it is a very compelling thought. If say the expansion slots were modular as well, you could put a PCI-e 3.0 slot, 2x spacers or PCI-e x1 slots, then another 3.0 slot. This would let those doing SLI/Crossfire put as much space as they like between cards as that is often an issue.
Another potential option is the RAM slots, if you start with 2 slots that max at 16gb DDR4, then for whatever reason want more, you could buy a module with 4 slots and max it at 32gb, instead of buying a while new board. (just as example).
Imagine being able to go from a socket 1151 CPU to a 2011v3 CPU, just by buying a couple modules.
There would be limits though, like when RAM standards change. Since all CPU's have an IMC now, you would have to stick to RAM modules compatible with that IMC. So upgrading from DDR4 to DDR5, would still require more than a single module change.