My suggestion for a new type of mission offered by traders is:
Invasion: ''Small waves of zombies will spawn inside the mission's area overtime throughout the mission walkthrough and will stop as soon as all the mission's objective(s) is/are completed. (For example: a small wave of zombies spawning every 5 minutes that have passed since the start of the mission.)''
This kind of mission would of course give a bit the feeling of a blood moon, but in a different context, adding pressure and challenge to the player(s) completing that particular type of mission. This type of mission, just like the 'Infestation' one, would also be equivalent to an additional mission tier in difficulty.