Isn't this identical to upgradable zombie survivals? Yes, they usually require some admin/game controller who can be abusive, but the fact is that it can be automated with some effort, like Aplem's Updgradable Zombie (Bot events) Survival
The gamemode that we were originally going to make was a zombie defense game (what you are describing), and the mod would essentially just be an AI director who took the place of the guy in the control room. However, after seeing the epic new horde mode, we decided to model our gamemode after that.
And don't give Aplem too much credit. Rykuta and I hosted Zombie Defense games back in v9, maybe even v8