You are given an opportunity to hard reset our universe (particularly, the Earth). You are also allowed to change one rule about the new universe. What rule would you change?
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.
All sentient beings have photographic memories and, through DNA, they can store and recall all of their previous ancestors' memories. All the way back to their oldest sentient ancestor.