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.
Wonder what would be the advantages if it has mass?
The universe would have collapsed back into itself shortly after the big bang, which would avoid a lot of suffering.
laughs and giggles?