just came to my mind: What about a daily (coin) reward for the players being first place on high score?
Maybe something like that:
- 1st for a period of n in normal, gives all players 1 hero coin
- 1st for a period of n in hard, gives all players 2 hero coin
- 1st for a period of n in nightmare, gives all players 3 hero coin
So, playing through and reaching the top can be a team vs team competition outside the map not disturbing the co-op spirit of the game, but create a new element in the lobby.