Leviathan gets two thumbs up from me. Dug it a good bit. Cool story beats and some sweet new scenery. There's some pretty obvious padding, but I found it easily forgivable. Haven't re-beat the game, so I can't speak to that effect.

It would seem based on my playthrough that only Conspiracy Theorist is a missable achievement, for what it's worth. Interested for others to weigh in.

Also, after playing so much multiplayer, the single player is even easier than it was before.