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.
No more Pauli exclusion principle. I’m not smart enough to describe what that world would look like… which is exactly why I would do it.
I’m pretty sure that would result in all matter compacting down to a single point again…