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.
It would change the topographical nature of the universe. We would probably have to exist in like the 3.1415th dimension or something to make it work.
Fuck it, I’m in. Can’t be any worse than regular 3 dimensions.