Adding on to this but I'm not sure if it's 1:1 what you're talking about.

PokeMMO is a online Pokemon Fangame that combines the first 5 generations of games. From what I gather, this is possible because it is up to the user to provide the ROMs, so litigious Nintendo cannot say they are re-distributing copyrighted material

Does it only use the assets from the original games, or also the scripting? If the former, then I'd say it's basically the same concept that I'm talking about, but with making a new game using the existing assets rather than reimplementing an existing one. If it uses the scripting as well and then provides some extra stuff to merge them and put it online, I'd say it's a slightly different (but still extremely cool!) thing.