I genuinely think a new unique gamemode would be good for people who play Zombie massacre quite a lot. Something that give players a good challenge and a lot more replayability.
My idea of a hardcore mode would start you at one life and one life only and reviving can only be done by other players. Classes would be nerfed to a certain extent (ex: Doctor’s Healing kit would have its healing affect reduced). A unique map specifically for the gamemode would also make it even more unique. The gamemode would be extremely difficult however the unit pay would be increased to 1.5x the amount compared to the 1.3x amount you get from very hard difficulty. Achievements could also be added for completing the difficulty altogether and completing the gamemode without ever dying.