An average programmer can probably get an emulator for that running in a weekend, and there are lots of guides and documetnation out there for the opcodes and similar.
NES / Gameboy / GameGear / similar are probably well-documented, but they will be harder at least because the processors have more opcodes you have to care about.
Yeah CHIP8 is a simple enough system that i even made two emulators (one in DOS and one in Windows) and an assembler back in 1999-2000 or so when i was in highschool and could barely put eight bits together to form a byte.
Also somehow for some reason i was convinced back then that mobile phones in the future will have a CHIP8 emulator to play games :-P. Sadly(?) they got Java instead.
I would say this is a good place to start. I did a Game Gear emulator in a single night with a buddy, having never written one before, just for the lulz.
You are bound to have an awesome game you can find on an 8-bit system and it's really fun trying to get to the point where you can play the game: getting the logo to appear, getting the title screen up, getting the sprites working, adding controls etc.
