mosh pit

How about having a mosh pit. Small arena with 5 players vs monsters. After every 25 monster, a boss spawns. The level of difficulty increases as you progress. There can be achievements associated with this and/or a stat showing the longest match that you've lasted.

The match can go on for infinitely long until the monsters are impossible. Also multiple bosses can spawn at the later stages. This will encourage teamwork at the later stages. Players can join in at any time until a certain stage.

maybe there can also be some loot from the bosses (of course depending on the level of difficulty). This way players won't get bored with the mosh pit and have some satisfaction.