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.
“Let’s see what scientists say about time if I remove entropy from the equation.”
Also: Big Elden Ring vibes. This is basically what Marika does; alters reality and takes death out of it because she got access to the source code of the universe.