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.
Around the time the pyramid of Giza is built, actual Aliens build a slightly smaller pyramid in modern day Tunisia. No one cares and all conspiracies still center on the ancient egyptian ones.