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.
In that scenario, how do you get established borders in the first place?
I thought on this also, and I suppose in my “rule” to adhere by I would have to choose a time in human history. I’d likely choose a time from year 0-1000 but would need to research and get input from folks smarter than I before choosing the time that rule would be implemented.
Of course, it only bans colonization. This is specifically the act of taking over lands from those who are indigenous to said lands, usually by force. If the governments of two lands what to trade or sell their lands in agreement without force, this would not be illegal.
I’m certain humans would still find a way to be horrible in some other new fun way.