Crafting potions/magic items rules. I get that there is a benefit to these rules not existing so the DM can determine it, but that tends to just make more work for the DM than encourage creativity, IME. Potions of Fire Breath could require red dragon blood, giantslayer weapons could require ground up giant bones, and so on. They don't have to do all of them, but give some more guidelines than just "the DM determines it".
More prices for typical items, like wood, clay, building houses, the costs of a whole town living, etc. Just more rules for taking care of a home base in general.
Better unconsciousness/0 hp rules. When you're reduced to 0 hp, you shouldn't just immediately fall unconscious, you should just be incapable of fighting, possibly staggering on the ground trying to get up. Currently it's very immersion breaking that a troll bites you, so you fall asleep, and then the cleric prays, and you wake up, and then the cycle keeps going until one side is dead.