Capability of microcode hacking?

Intel has been releasing microcode updates for its CPUs since at least January 29, 2000. AMD has been releasing microcode updates for a similar length of time.

In theory, it's possible to reprogram an Intel chip to do whatever you want, within the capabilities of the silicon. In practice, as the above website notes, microcode updates appear to be signed using best-practices public-key cryptography. As a result, only microcode updates signed by Intel will be accepted by the chip: neither tinkerers nor malicious actors can provide a valid update.

To express it clearer: the limiting factor of that will most probably be the "capabilities of the silicon" - beside the available possibilities to break the mentioned cryptography, as the available resources (microcode store area and hidden registers) in most cases should be very linited.

Otherwise opening the micro instruction set and marketing the processor as a (new) "RISC processor with x86 emulation software" could be much more promising.