From what Square-Enix has hinted about the Dark Knight, in that it will not have a base class and that you will be able to play it right away if you buy the expansion (they didn't say you had to have an existing character at 50 or anything), I almost wonder if it could be a new job that starts in Ishgard right at some mid-level, like maybe 40 or even 50. If you start a new character on this job, you would jump right into the 3.0 storyline without going through the 2.0 story path. For example, you could be a Dark Knight that joins the Scions once Ishgard finally opens its gates. Of course it would also be possible to take an existing 50 or whatever and learn this job, and take one of these characters and learn the other jobs, which would progress as usual.

This approach would have some controversy, but because Dark Knight is a tank class, S-E might feel that such an approach would bring more tanks into end-game more quickly. And given the progression in the story, and since you can play them right away with the expansion, I doubt it makes sense story-wise to have them start at level 1 in some other city and go through the whole 2.0 path.

I guess we'll see how it plays out, but if so, Ishgard could be a new starting city-state, but in a different sense. (And if they do take this approach, it would make sense for the other new job to be a healer-type, because then they have an influx of healers and tanks in end-game more quickly.)