I’ve thought about making an AVR-8 emulator in AVR-8 assembly language that would let me run tiny programs out of RAM where the emulator (host) would use some of the registers and the others would be be available to the guest. This way you could upload a function over the serial port and run it. I figured it would be a good way to really master AVR-8 assembly.

The ESP32 has a huge amount of potential for emulating other things, see

https://github.com/breakintoprogram/agon-vdp

For a display controller implemented for ESP32.