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.
Gravity doesn’t exist. All matter interactions are by electromagnetism and Ver Der Waals forces only.
Granted. Matter never collects in a way that would allow for life to be created.
Objective accomplished.
I was going to say this but for the weak nuclear force.