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.
Fusion makes a bomb like the sun.
Fission makes a bomb like chernobyl.
Seems similar, actually different. We’re actually pretty good at fission, but fusion is way harder. The problem with fusion is that you need a huge amount of power to generate a slightly huger amount of power in return, and we are pretty crap at generating a big enough spike of energy to start the reaction and marginally worse at capturing said power after. All in all, pretty far away from practical fusion power.
Fission, by comparison, can happen when you have too many special minerals in a wrong shaped pile. We abuse this effect to boil water, and use that boiling water to make power.