if anyones played the grand theft auto series and modded it, they probably know what im talking about. if not, well, what a chaos mod essentially entails is, on a timer that can be customised, a random effect happens to the game, which can either happen instantly or happen during a set duration until it ends. effects can range from being helpful, hindering, or merely cosmetic.

some ideas for effects i had (you could get as ridiculous as you want)

-remove all of players weapons
-instant max wanted level
-replace all npcs with aggressive elephants
-make everyone on the map's heads explode
-give napalm piss for a duration
-spawn aggressive krotchy
-force suicide
and more.

i honestly think postal 2 could be amazing for that. id mod it myself but too many resources on that modding page seem to be deprecated due to age so i cant really be bothered looking through for that stuff. but yeah, i think itd be pretty fun.