I just finished watching ATLA a few days ago and am almost done with TLOK Season 1 (I’m just about to watch “Endgame” as I’m writing this) but because of spoilers I read at TvTropes, I know about the Kosira ship. From what I understand, each season/Book having its own arc was a deliberate decision by the show’s creators to highlight how the world needs the Avatar to deal with its many issues and problems, as opposed to ATLA’s single Big Bad. More commentary on TLOK from me will have to wait until I finish watching the rest of Books 2 through 4.
EDIT: Just finished watching “Endgame“ and reading TvTropes’ recap of the episode. Apparently, Book 1 was originally intended to be a single season “miniseries” before Nickelodeon approved the later seasons. (If this fact was mentioned in an earlier post, I missed it. Derp.) Frankly, the scene of Amon+Tarrlok+electric glove+speedboat fuel tank was a Wham Shot for me. I officially have NO idea how Nickelodeon let the writers get away with that on a kid’s show. Still a Tear Jerker/Alas, Poor Villain moment, though.