Bonus round idea.

What if instead of having a bonus wave you gave secondary objectives during wave 16? For example everyone gets bonus points if the lowest level player is still alive at the end of the wave. Something like that would be hard to actually accomplish but would promote teamwork. The only drawback would be if people started complaining about loosing the bonus points when a new player joined or something like that.

Another objective could be awarding bonus xp for killing a certain monster(s) during the wave. If you could configure it to spawn only one of these monsters you could give it a new skin add on some health or whatever. Or you could have an award for the player who kills the most nali cows or pupa ect... I guess that one could be per wave or per map.