The rule could be anything, as funny or as serious as you want. The universe will progress in a similar way that it has up until this point, unless your changed rule prevented it from doing so.
Some examples might be:
- The invention of currency is not allowed.
- Iron is slightly less stable.
- The Ancient Greeks are able to cultivate Silphium, which does not go extinct now.
Greed is removed from the list of possible emotions/personality traits.
Great. Now everyone’s horny.
You say that like it wasn’t intended.
Welcome to my world.
That was gonna be my change
Everyone horny all the time
So bonobos become the dominant species?
I’m already in that universe; it’s pretty good
Nobody is horny and humanity dies hornyness = greed for sex.