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.
If you eat a piece of cheese, you get to levitate for 10 seconds no more, no less. Doesn’t matter the size of the cheese or the type (this includes lactose free).
Would be nice for the nervous dogs in the world.