Forgotten Sanctum is by far my favorite DLC. While less innovative than either Beast of Winter or Seeker/Slayer/Survivor it's far more polished, and really adds where the game needs it. The dungeon it adds feels less linear than BoW, with multiple side rooms and a few good choices with meaningful consequences on the way through. There were lots of different ways to solve different problems, but some neat fights too.
The writing was much more consistent in this one too. Beast of Winter felt too short, and Seeker / Slayer / Survivor was not really writing-focused. Focusing on three main npcs provides some much better options for characterization. Tayn really steals the show. He's a wonderful mix as both profoundly unreliable, but possibly benevolent and surprisingly competent. I really hope Tayn is a party member in the next game. They also expanded two of the most interesting sidekicks. Fassima is a bit weak mechanically, but narratively she's pretty interesting.
The best part of the writing was the expansion of the lore and many, many hooks for the next game. There's the potential stirrings of apocalypse (not feeling great about reforging the Crown of Woedica). There's the Yezhua. The dlc introduced potential of creator gods (which seems doubtful). Wilder histories we don't know about (Orlans were considered wilder in that). The potential for Apotheosis, given that Skaen used to be human. And best of all, it seems like there's one missing titan body. Three weren't there. We've seen Abydon's and Wael's. The fact that Eothas had to possess Waiden makes him unlikely. I think the obvious candidate is Magran, but who knows?
Finally, it presents you with a clear, meaningful choice at the end.
The only complaints I have were the bosses. The laserbeam fight sucked in a game with disengagement. All of the bosses felt a bit hp bloated - watching your empowered missile storm only ship only chip away a fraction of health is disheartening. Also, I really wish they would stop adding mobs in the beginning of a boss fight. It removes tactical decisions in a way; why do anything but rush the boss if there's an infinite swarm jumping in behind him?
All in all, I was pretty happy with the expansion and look forward to seeing more work of this quality from Obsidian in the future.