I would like to suggest a challenge that would be a fun addition to PvP or even as a co-op. The players would be in a room and the goal is to take out as many monsters as possible in a time frame or to see how long they can last. The players would endure waves of monsters, increasing with difficulty with health drops and vials to aid in survival. Picture this similar to Boxhead. It's about surviving as long as possible or to the end depending on how it's developed.