Mhmm, this is great! someone should do something like this to create a "Mario 35" clone: Create the game based in the original ROM and then setup a page with a "diff" file that people can use to recreate it if they have the original ROM and game.
https://www.vice.com/en/article/kzmykz/the-creator-of-mario-...
E.g. see Project Slippi[1], a patch for playing Melee online with rollback netcode.
Interestingly, a tournament using was Slippi was issued a C&D[2], but Nintendo has issued C&Ds to Melee tournaments in the past just for using their graphics and making money and stuff.
[1]: https://slippi.gg/
[2]: https://twitter.com/TheBigHouseSSB/status/132952108157785703...