Well, Yoshi-P said that as soon as you register Heavensward, you'll be able to play the Dark Knight. To me, that implies new players, as well. You won't be able to play a Dark Knight without the expansion, thus wouldn't be able to start in Ishgard, either.

The alternative is... what, assign the new Jobs to existing City-States? I could see Machinist fitting in with Ul'dah or Limsa Lominsa, but the other two, I wouldn't imagine so. I know the prevailing theory is that you'll have to be level 50 and travel to Ishgard to acquire the new Jobs, but I honestly think we'd have heard something about that if it were indeed the case. FFXIV is quite a lot different than FFXI -- the fact we're getting a new Race at all proves that -- and I don't see them locking out the new Jobs, which are a major selling-point of the expansion -- from new players.

A personal anecdote was when I purchased FFXI and several of its expansions years ago, I was horrified when I learned I couldn't access the Classes I'd paid for, until I'd already gotten high enough level to unlock them. I quickly lost interest with the game, and I often wonder if those Classes had been immediately playable, if I might have stuck with it longer.