If you could change one rule of your native language (grammar, syntax, spelling, etc.), what would it be?

I would remove gendered things like different pronouns per gender and things like "lioness" or "waitress". People then must specify "male" or "female" "lion" or "waiter", if that's significant.
- The other I learned that a group of lions is called "a pride". Sounds cool, but quite difficult to learn all the group nouns for all the animals as a non-native
  
  I second this. Even native speakers are often ignorant of all the possibilities. Just call everything a herd.

Introduce a 4th grammatical gender and, randomly, change 1/4th of all nouns to use the new one.

I would get rid of the formal "Sie" and change it to the informal "Du". No need for the two to coexist. Sie/Du is german for "You".
- How open is "Sie" still used on a day-to-day basis? Like do people use "Sie" for their boss?
  
  We use "Sie" for adult people we don't know and at work for people like the boss etc. Online we only use "Du", except in formal emails. But there's a lot of exceptions to these rules. I try to use "Du" wherever I can. When younger people call me "Sie" I immediately offer them the "Du".

I have a few I would change, but the first change I'd make is to add a few regarding identification in certain situations.

I'd put the adjectives after the noun, instead of before, e.g. "wagon red" instead of "red wagon".
"It was a big bright yellow triangular..."
Just tell me what the noun is! You can tell me more details later :)