If I were writing the D&D rulebook I’d make a Rule Negative One, right before Rule Zero (“The DM always has the authority to change the rules”).
The always and never rule:
In this book, the words “always” and “never” have a special meaning. Whenever you see these words, mentally add “with at least one interesting exception, to be determined by the players and DM.”
This rule always applies.
Then I’d make sure to use the words “always” and “never” a lot. I’d throw in tons of restrictions. Demons, drow, and orcs are always evil. Paladins are always lawful good. Dwarves are never wizards. Skeletons are always mindless. Under a Zone of Truth spell, you can never lie. Halflings can never resist a pie.