Oh, man, so many things. Remember, you asked! :)
First off, treasure hunting sounds freaking awesome, and is a classic example of good strategy games forcing you to make interesting choices. Do you spend time hunting treasure, or do you build up settlement again sooner? The choose-your-own adventure aspect of the sea and land treasure hunts sound like an incredibly satisfying addition to the gameplay.
Bridge battles! Fortress Gates! Ocean Hazards! All good stuff, potentially, without taking away from existing systems.
I love that settlements have more slots for building, and until this article pointed it out didn’t realize that would not only make planning easier for me, but for the AI as well. Always a good thing.
Intervention forces, mercs you can hire to harrass the enemy during their rituals, sound fun.
I adore the new UI changes, from being able to customize the things you see and the game pauses on during AI turns to the change in the movement bar. All this stuff was detailed in a video I shared above, and I highly recommend watching that if you haven’t.
The trait system sounds like something I’m going to get a lot of enjoyment out of, too. What a cool idea.
I mentioned it before but it bears repeating, I think the new map and the way you transition to the high-level map looks great. I think it’s crossed “handy” and into “amazing” territory that you can press Spacebar to bring up the overlay screen while in the normal campaign view to turn on the colored filters without having to zoom out (or go to a separate map).
The faction mechanics for all four of the races are appealing to me in some way or another - all of them sound like a faction I’ll enjoy playing as or against, which is a great thing I am always a little worried about with strategy games.
The combined map is sounding more and more amazing, with a lot of the above changes being retroactively added to the original game’s factions for the combined map.
I think moving away form mini-campaigns and having more “meat” for the race packs will be smart, as well.
That’s just re-skimming it, at least, but you get the idea. Some really bold and well thought out moves, in my opinion.