I had an idea while playing Arma 3 so I thought I'd throw it out there. It was an MMO survival game similar to H.G. Well's book War of the Worlds. DayZ is fun, but the zombies are not really a threat so people run around and become the actual threat. An alien invasion of earth, however, would force teamwork and still have some PvP because of the threat of a more advanced enemy. Just imagine seeing giant tripods in the distance destroying an entire city looking for players to destroy, using spotlights to try and find people.