I've made a mod in which killing a zombie spawns two zombies nearby after a short delay unless the original zombie was on fire. There are options for fire, delay and whether or not the zombies spawn nearby or directly on the corpse.
In theory this could lead to some really interesting gameplay where you really have to think about whether it's worth killing a zombie now only to have to deal with two more in a few seconds time or whether or not to use your last Molotov on a horde. If you have any suggestions or feedback then please do voice them so that I can improve the mod.
This works on IWBUMS (a basic build 23 version is also available).
To install all one must do is put the zip into their Username/Zomboid/mods/ folder, right-click it and select "Extract All" in the drop-down menu.