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.
Is hard to pick one.
Altruism is the default instead of selfishness
Cancer doesn’t exist
No harm could come to children. No one would have wicked thoughts towards them. They’d fully recover from any non-lethal injury. They could get sick.