why not create a list of ideas for games that could be realized in future? We can help programmers in concepts or ports and they will develop homebrews based in which the gamers like and dislike. We could post here for consult in future.
I think the Master System sorely lack beat 'em up games for two players, most of the ones that are available are for one player only. The only two players beat 'em up games are Double Dragon and Battletoads in Battlemaniacs (which is not even a full beat 'em up actually). Therefore I think the Master System should have some additional games which would cover that.
Some good options are actually on the NES and within those particular series actually: Double Dragon II on the NES it's a great two players beat 'em up and the best of the series (much better than the version of Double Dragon that the Master System got) and both Battletoads and Battletoads & Double Dragon on the NES are great choices in that regard too, especially the spin-off with Double Dragon which is mostly a beat 'em up game this time.
There's also the fact that Ys: The Vanished Omens never got its sequel for the Master System, Ys II: The Final Chapter, and lacks the conclusion of the story of the first game. But I guess this one would be harder to develop.