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.
Stupid thoughts become painful.
"It’s not painful bro you just need to feel it for longer you’ll get used to it. Ow.
Unfortunately, you didn’t get rid of masochism, so now they’re stupid and cumming.