Well, 1-15 could probably be fit in a single zone, in all honesty. Not saying that makes it trivial to create, but it's a fairly normal part of most MMO Expansions. And to me, getting that "tease" of Ishgard is why I love the idea so much. Dark Knights clearly just don't exist in Eorzea right now, so I like the idea of starting one and getting a feel for Ishgard's snowy environments, versus just introducing them in an existing city might seem strange (it worked for Rogues and Ninjas, because of who they are, but even that felt a tad forced).

They could even make it a cool story-hook. Have your character start off in a small Au Ra settlement, learning the ways of the new Jobs. Through your quests, you eventually uncover a corrupt member of the Ishgard religious order, and it leads to you fighting him. In order to protect both yourself and the settlement, you're instructed to leave, and told that the Scions of the Seventh Dawn could protect and make use of an adventurer like yourself.