I interpreted the Zombie Defense as some classic tower-defense style game-mode. Stationary object that has a limited HP that you must protect, and survivors can pool together upgrades to better protect that object, like more HP or turrets.
As for the waves, I feel like it'd be nice to have waves pass on a timer basis. Maybe waves could spawn "themes" of mobs? You might get exclusively Tickers, or maybe a pair of Skull Champions with no other zombies to act as a boss wave? Maybe the game could throw out everyone as a normal zombie with dozens upon dozens of AI normal zombies also spawning? It could be fun, and still act differently than PTK.