Even if extracting NES game data is not your thing, the emulator concept [Matthew] uses might be still worth a read.

You can create music levels that play songs, puzzle levels that switch up the traditional roles of Mario and his nemesis Bowser, homages to other games, and much, much more. There is one constant ...

Nintendo has detailed the final major content update for Super Mario Maker 2, and it will add the ability to create complete game worlds with the user's levels, in addition to new items and power-ups.