There are actually a good numer of forks, but not sure if they qualify for your criteria of "serious" or not. I also won't be naming any of them because those

There are actually a good numer of forks, but not sure if they qualify for your criteria of "serious" or not. I also won't be naming any of them because those "committed" mame devs are very quick to inject themselves into any story about them, and harshly judge everything else that touches their code that didn't come from them.