Rimworld

You don't have any? Do you feel like you have decent grasp of the base game? If not, I wouldn't worry about adding anything extra yet. They all add significantly more complexity and that's probably not good if you're still trying to internalize the core mechanisms.

If you're ready for an expansion, I would recommend Royalty first. It adds more types of missions, royal titles, psychic powers, and new gear (clothing, armor, weapons, etc.).