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.
I think we are in the same page, I am just so sick of people confusing theory with hypothesis. Mostly because of christomaniac creationists saying “uhm actually evolution is a theory not a law” and shit like thar. That’s the reason why I rejected OP’s statement