Dawntrail's themes are actually legacy (the JP name for the expansion is, flat-out, "Gilded Legacy"), family, and the weight of expectations.
The thread of looking to the future with hope instead of attempting to bring back what was lost at any cost, a common theme from 2.0 onward, is there, but it's kind of a knock-on because the story features rebirth as such a prominent motif. (Phoenix is basically CS III's unofficial mascot.)