First, I didn't realize there was a need for more slots, more memory, etc. Are you having a problem finding a motherboard with enough slots and memory for your usage?
Second, they have already done this years ago to a certain degree, with CPU's being mounted on cards that plugged into a slot on the board, remember slot A and Slot 1 and Slot 2 CPU's?
For many different reasons, as the dies shrank, and as CPU's became faster and faster, putting them back on the board was the optimal solution for performance.
While it would be neat to see something like that, engineering and technical requirements would tough to achieve a device that performed as well as having the CPU on the mainboard. But, in 10 years, the CPU's we think of today may become completely obsolete and running under a different technology all together.