It is a notion I have contemplated before. However I am not sure presenting someone with the purest example of utterly basic and generic ROM, one that allows you to experience the basics of table making/text editing without nasty compression or oddities breaking your flow (repeat this with equivalents for graphics, game logic, character stats, level formats and whatever else), will do all that much good. Likewise chucking you in at the deep end is unlikely to do much either.
To that end one for a game you are interested in and think you can usefully make an improvement for (give me any game and I will give you an improvement for it, however if for one given game that is going to see me redo half the code, graphics and music then that is going to be many thousands of hours and probably learning a few more skills it becomes unviable).
If you want to pick a game that already is well known (megaman, pokemon, advance wars, zelda, various mario titles, sonic, final fantasy, fire emblem, various other square/square enix efforts... https://datacrystal.romhacking.net/
and the documents here as well as dedicated sites and the like for each of those) and have whatever is already known as a reference in case you get stuck and just need to move onto the next thing (or having the answer figure out how you would get to it) then that might also be useful.
If you want to make it a nicer English language (or whatever language you know) rather than some hardcore Japanese thing then probably for the best. You need not make something that some future hacker can use either -- if the game has some punctuation with odd encodings and does not use it in the game as long as you say brute forced one or two you don't need to do them all. Make sure you learn to make cheats as well.