This already exists on the SNES with the usb2snes/SNI family of protocols. On original hardware, it requires a flashcart that can sniff the memory bus and interface with a host PC over USB.
Some things you can do with this include:
- RetroAchievements, as you mentioned
- Automatic timers/splitters for speedrunning
- Co-op mode for Super Metroid: https://m.youtube.com/watch?v=ilG70fc-IJA
- Multiworld co-op randomizers for games like Super Metroid or A Link to the Past -- two or more people play linked randomizer seeds over the Internet, and the items are distributed across games so you can find items for other players in your game: https://archipelago.gg/
Some things you can do with this include: - RetroAchievements, as you mentioned - Automatic timers/splitters for speedrunning - Co-op mode for Super Metroid: https://m.youtube.com/watch?v=ilG70fc-IJA - Multiworld co-op randomizers for games like Super Metroid or A Link to the Past -- two or more people play linked randomizer seeds over the Internet, and the items are distributed across games so you can find items for other players in your game: https://archipelago.gg/