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.
Fell asleep thinking of a question similar last night. No colonialism. Colonizing other lands outside of your own established borders would be forbidden.
How different the world would be.
In that scenario, how do you get established borders in the first place?