It was amazing with JE. My first play through, I didn't see it coming at all... I actually thought I'd beaten the game just before it happened.

The second play through, I was amazed that I hadn't seen it coming, as there were many hints.
How about this: Joinable NPC who romances you then betrays you... only to regret or not regret their choice with numerous possible outcomes based on decision taken by Hawke prior to betrayal. Hasn't been done yet.
While I didn't see Morrigan's offer coming in DAO, before she started speaking in Redcliffe, I was expecting a full-on battle with her (despite having romanced her), and was somewhat disappointed to see that it was merely an offer. I mean, a huge battle with your LI, followed by possible reconcilation or not depending on prior choices, with huge implications for the ending---that would be super.
jackkel dragon wrote...
I have to say I'd like a Jade Empire-like reveal in DA2. I'm not as keen on the KOTOR twist, but that doesn't seem like an option from what we've learned so far. But in Jade Empire, there were hints along the way (great for replays) and the actual twist comes almost out of the blue. (Having been spoiled, and noticing the spoilerific "chapter summary," it was mostly when it was going to happen that was new, and it still got me by surprise.)