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.
Every time you’re unreasonably rude to someone, you feel a pinprick inside your nostril. If you’re a right jerk it’s more of a stab. When you assault someone the pin is a kitchen knife. Murder and rape make you feel a lance of fire in your sinuses. It could work, nothing else has