Things like that existed in the category of accelerator cards. Xeon Phi (Knights) is one example, focused on core count. Some from HP have soldered on SSDs too. You also had blade servers which is more focused on that use case, though that's going out of style.
I don't think PCIe is really a good fit for general CPU tasks. You need big heatsinks and power and can't fit that much RAM on board.
I don't think PCIe is really a good fit for general CPU tasks. You need big heatsinks and power and can't fit that much RAM on board.