Does your friend want a complicated expansion strategy-wise? Complicated cards? Or do they want a lot of interesting new mechanics? The answer depends.
If they want all three, I would recommend Empires or Adventures.
If they want complex strategies and complex cards without introducing new mechanics, I would recommend Dark Ages.
If they want complex strategies and a lot of new mechanics, but cards which are still easy, I would recommend Renaissance (Menagerie may also fit in here, from the few games I have played).
If they just want complex strategy, I would recommend Prosperity or Seaside (these also add new mechanics, but not as many of them).
I know I didn't include Nocturne here. If I included it, I would put it with Empires and Adventures. However, I personally think it has too much complexity density, and Hexes and Boons are my two least favorite mechanics. It's still fun, but I wouldn't buy it as a first expansion for anyone. I also didn't include any of the expansions which you ruled out (except for Dark Ages, because I think it has the largest strategy complexity / new mechanics ratio of any expansion, and it's really fun).