Here is a ROM editor for Metroid. It is incomplete at the moment, but it’s fun to play with for now.

Have fun until it actually starts working!

I’m not sure if you are allowed to post about this because ROMS are illigal.

No actual ROMs there, so it’s fine.

Unless I missed one in my quick skim.

None. Even when it becomes final, you have to load in your own ROM for it to save.