I don't think there is any game that needs to be a one-off as long as there is a good idea for a sequel. There doesn't seem to be a good idea for Bioshock 2. Like, consider some movie sequels like Terminator 2 or Aliens. Terminator 1 and Alien were thriller movies rather than the more action movie sequels. They are set apart from the original movies despite having a lot of similarities. Bioshock 2 didn't need to be Bioshock again. The ending was so open that they could have had a better idea than "so, you're a big daddy in rapture and there is a new leader with a different political view". Maybe they switch things up on you at some point in the game and it will become really awesome at that point. This is all just based on what I've seen so far and all I think of it is how unambitious it seems. Realistically I'm hating on it a bit more because I keep thinking there was a Bioshock 2 that 'could have been' I would have been excited for. I liked Bioshock a lot and this sequel does very little for me. I'm always open to being proven wrong when it's released.
Bioshock 2 should have been: Andrew Ryan survives, becomes a drunk, grows a beard, and wanders around an empty rapture beating up the physical manifestations of his drinking problem not knowing if what he's fighting is in his head or just the homeless people who stuck around. He does this with his fist. There is also a Bear loose in Rapture who wears Big Daddy armor.