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.
Lol, I wanted to break this law so that we can have infinite energy sources (fun stuff) and to be able to destroy ~useless energy (like thermal energy which is a byproduct of probably all energy conversions), thus reducing entropy and other fun stuff.
Yeah I was just teasing you. That actually seems like one of the more fun ones here… Though there’s some frightening implications.