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.
Every living creature is entitled to an equal portion of useful surface area of the earth proportional to their size and space requirements.
Not like that has anything to do with us possibly living out of a car next year if rent rises 30% again for the 3rd year running.
Fuck landlords hoarding property.