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.
Nah we definitely have fusion, just not for anything other than bombs.
Fun fact: to set off a fusion reaction, hydrogen bombs actually have a smaller fission bomb inside it. Sometimes multiples of fission-fusion reactions all stacked inside of eachother like nesting dolls.