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.
Yes, gravity is a theory. In scientific terms, theories are proven, repeatable, and accepted. It’s the most robust and strongest form of scientific “fact” we have (since new discoveries can change our understanding, we can’t honestly declare it indisputably factual).
When people say “I have a theory…” they usually mean “I have a hypothesis…” which is some idea or problem statement that is unproven, untested, unverified.
OP was playing off the conflation of those two things.
I think we are in the same page, I am just so sick of people confusing theory with hypothesis. Mostly because of christomaniac creationists saying “uhm actually evolution is a theory not a law” and shit like thar. That’s the reason why I rejected OP’s statement