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.
    • bizarroland
      9•22 days ago

      It would change the topographical nature of the universe. We would probably have to exist in like the 3.1415th dimension or something to make it work.