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.
One free wish for any and every person who achieves something truly outstanding for mankind as whole (only positive achievements).
Maybe also for a free “weaker” wish for personal achievements, like completing one’s dream hobby project.