I don’t know the correct term so I’m stealing the one from payday 2. And if I stole your idea, apologies.
Essentially a set of extra challenges in the form of boxes to check when you start up a lobby. This could make it so that all zombies move twice as fast, the boss can only be hurt with melee, every survivor has 1hp, no lives and no revives.
Stuff like this that will allow for a greater number of games that feel unique and means players won’t be stuck just playing the same 6-day system for three maps.
This could increase or decrease the multiplier for games or outright remove the unit multiplier altogether but that’s a bit too far thought ahead.