https://www.evilmadscientist.com/2016/6502/
is a minicomputer implementation of the 6502. You can also make a "soft core" than can be uploaded to an FPGA, see
https://en.wikipedia.org/wiki/Soft_microprocessor
Some of those are open source. Probably the simplest CPU architecture for the CPU hacker is
https://en.wikipedia.org/wiki/Transport_triggered_architectu...
which is not only easy to design, but lets you add custom processing units such that you can codesign the chip with the software it runs.
Enjoy :)