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.

I'd probably say mundane ergonomics (eg.: how do your species use chairs, or if doors have knobs or bars) and general interpersonal processes: how does one find a mate, or raise a child in the setting, how are liars and cheaters treated, etc.

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.