I haven't seen any other suggestions similar so here it is. I was thinking you could make servers that actually go till either the zombies overrun the players or the players prevail. There would have to be some kind of server sign up based on general time frames players could play and max number of players. The total number of zombies would also have to be fixed. If a player died in the server you could just ban them until the server resets- this would probably make players more willing to work together or not. Put a time limit ( would depend on how many players are on the server I guess, I was thinking somewhere around a week or month real time maybe or just no time limit and it just goes till one side is victor) on server and around end of time limit have any zombies that are left make their way towards the nearest survivors as a finale. My idea probably needs some work but I think it would be fun and very intense with the whole one life to lose aspect.