I don't know where exactly to ask this, i don't see a rom hacking general section, but would it be possible to hack donkey kong country games (or maybe even mario) to have 2 player on-screen multiplayer? Like contra and sonic the hedgehog.
Yes, it's possible, of course. In fact, something like this has already been done for Super Mario World. However, that doesn't mean it's easy to do. Even the Super Mario World hack isn't fully functional in all aspects and I actually doubt that someone will do a hack like that for Donkey Kong Country. At least not within the next few years.
You can find the Super Mario World Co-op on youtube,I saw it a while ago,but for some reason on the 4th stage the Goal bar was not there so we couldn't go any further.