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.
Base 36 counting becomes predominant.
Also the point at which hundred changes over to thousand gets pushed back by a power, so 6KL2 is Sixty Kay Hundred Elty Two, while 8,A59G is Eight Thousand Ayty Five Hundred Ninety G. Same with the changeover to Millions, Billions, Trillions, etc.
Probably wouldn’t change that much practically, but it’d make “Ten” a square that’s also the product of two prime squares, and it’s divisible by a lot more ways than the current number base, and it makes “One Thousand” the square of “One Hundred” the same way “One Hundred” is the square of “Ten” and that “One Million” is the square of “One Thousand.”
Okay but now you only have 3 fingers on each hand