"Play until you die" is great for single player but in private multiplayer servers, death means less. You don't want to stop at death because all of your friends are still in, but if you respawn you know it never ends.
To prevent this cycle that ends in burnout for some, a leaderboard and admin conditions could make for a clearer endgame. Who can survive the longest character in a year? Who can end the most zombies? Who can maintain the longest original character?
An in-game record of player bests could give players these endgame goals, while scripted win conditions could do even more. Any chance of seeing any of this?