I think adding the possibility for maps to be played by two players simultaneously would greatly increase multiplayer interest.
Basicly, it would mean that two beatmaps would be played at the same time, one for the each player. The players can then watch the other persons pointer and clicks etc etc. Colors would indicate which hit-circle belongs to which player. Of course, this would require beatmap-makers to make two beatmaps for one song. Beatmaps which support this could be tagged as "Multiplayer" the same way that storyboarded beatmaps are tagged today.
Basicly, it would mean that two beatmaps would be played at the same time, one for the each player. The players can then watch the other persons pointer and clicks etc etc. Colors would indicate which hit-circle belongs to which player. Of course, this would require beatmap-makers to make two beatmaps for one song. Beatmaps which support this could be tagged as "Multiplayer" the same way that storyboarded beatmaps are tagged today.