I suggest adding this slightly complicated game mode to AoS.
Compatible With:
-1 to 4 players
Description
The player would be stuck in a small 9x9x4 house (though people can expand it), and would have to defend their building against simple AI, which walk in a straight line and are intent to destroy the house. Each zombie killed gives the player 100 points, or 150 points if it was a headshot. After each wave is finished, there will be a 1.5 minute break and, players, with the help of a simple commandline, can get weapon upgrades (higher rate of firer, bigger magazine size, more accuracy, less recoil, bigger damage and so on), building upgrades( stronger blocks) and buy ammo, medkits (gives player 25 health per use, can store 3 at the same time max) and blocks (max of 25, can upgrade to 50). During the break, players are also able to switch weapons WITHOUT dying and repair their building. Zombies that take more bullets to kill, move faster, destroy blocks faster or (perhaps) spit out poison (every shot that hits the player deducts 40 health points) (all different types of zombies have different skins) are introduced as each wave is finished. The zombies are also able to 'spade' or 'claw' (1 claw every 1 second) the player if they get to close to them. The game ends when the entire building the player is in is destroyed or if the player is killed by poison spit, clawed to death or killed by their own stupidity.
Verdict (or tl:dr)
Player is stuck in a small blocky house and pitted with killer zombies that spawn outside the house. With every kill the player gets, money is earned. The money can be used to get upgrades and medkits. The zombies get more powerful after every wave.