I was just thinking about this a few days ago, but not just for the CPU (which we have RISC-V and OpenPOWER), but for an entire system, including the GPU, audio, disk controllers, networking, etc. I think a great target would be mid-2000s graphics and networking; I could go back to a 2006 Mac Pro without too much hardship. Having a fully-open equivalent to mid-2000s hardware would be a boon for open computing.

There's no point. The big chip makers control all the billion dollar fabs. Governments and corporations can easily dictate terms. We'll lose this battle unless we develop a way to cheaply fabricate chips in a garage.

The future is bleak.

Sounds like you might want to go play with RISC-V, either in hardware or emulation.