If you're looking to emulate another instruction set I hear the GameBoy or GameBoy Color isnt too hard. There's tons of documentation, examples, and it is still challenging.
I personally enjoy emulating old game servers.
I start digging into the game client and documenting the protocol used for sending packets around, spin up a basic server in whatever language seems best at the time, and start working at assembling systems for the games features as closely to what it seems the original developers did based off the client code.