I just finished finalizing the story for Twilight of the Arnor. While Dread Lords and Dark Avatar were interesting in their own ways, I think that this campaign will be the most fun and interesting of them all. Players will really get to learn about the Precursors and there will be a LOT of Dread Lord action in this. There are entire missions where it's just you vs. Dread Lords. It's no holds barred.
One interesting mechanic will be the addition of the new Immense sized galaxy which one of the campaign missions makes use of. With 400+ sectors in a given map, there will be plenty of action.
All players of GalCiv will also benefit from Twilight of the Arnor since we'll have to optimize a lot of code in order to make possible some of the things in Twilight. And some of those optimizations will be made available throughout all versions.
Hopefully people will like what we're doing with Twilight. It's going to play a lot differently in the campaign than the previous ones.