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.
Silphium was recently rediscovered so it’s no longer considered extinct just highly endangered.
Aside from that, I would make it so that instead of having mental illness you would have magical powers.
Schizophrenic? Nope you just talk to the Dead. Adhd? Nope, you’re an elemental wizard. Bipolar? Nope, you’re either a healer or a necromancer. Depressed? Nope, you’re just low on mana. (Major depression means that you’re suffering from a curse) Psychotic? Nope, you’re suffering a mana storm Narcissistic? Nope, you are an illusionist. Autistic? Nope, you’re an enchanter.
Maybe instead of schizophrenics talking to the dead they would just fit into the class of spiritualists and diviners, oracles and the like.
I’m open to interpretation.
After a little more thinking on this:
Gender dysphoria? Nope! Shapeshifter. Furry? Nope Werebeing
Basically, anything that is a deviation from bog standard human would have a magical reasoning behind it.