I don't know if anyone has brought this or not but I feel like adding more game modes would increase the player bases. On top of that it would defeat the repetitiveness that some complain about. As I am not a fan of PvP I think I would enjoy a sort of "Fire fight" type of game mode where its just endless waves of enemies and after like 5 or 10 waves a random boss will appear. Eventually adding in challenges as well like more elite spawns, increase enemy damage, speed boost, etc. Just random challenges that make the waves harder.
I feel like this type of game mode would be fun although should be able to be played solo as well. Although I don't fully know how the boss fights will work sense most bosses like Aneska has landing spots on the map but I have faith that MSE can make it work. If they even consider it.