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 born
immediate ptsd from the memory of my parents plowing passionately which ended with my creation
crawls to the roof and jumps
It wouldn’t be weird if you just always knew