Magic systems.
Not the spells, but how and why magic works in the first place.
For non magic worlds, it's creating unique "races". For my home brew ttrpg system and world, I've come up with a few that I'm extra proud of that I've never seen anything like. There's one that I built from the ground up for my favorite player (shhh, DMs do have favorites, but it's a secret) based off of a picture in a magazine, which isn't totally unique because of that, but it's still one of my better interactive bits in that world.
Has anyone found a better author at designing magic systems than Brandon Sanderson? He's the best that I've come across.
Not that I know of for sure
Calendar and astrology development. The true power fantasy of fantasy worldbuilding is devising a world with a consistent number of days in a month...
Sounds fiddly lol. Not to knock it, everyone has their thing, I just don't think I've heard that one before.
i always loved the shire calendar in the indices of the lord of the rings. it's just so neat and tidy.
I love designing the geology. Mountain ranges, oceans, plate tectonics (when applicable), etc.
i always like thinking about the everyday cultural stuff. art forms, foods, music, attire, and so on.
