Unlocking Dark Knight?

[Xenosan]

They discussed it in the last live letter and benchmark release. You don't need Ishgard access to be a DRK/MAC/AST, you can buy a sword/cards/gun anywhere (or get drops from HM Primals; they don't have a zodiac weapons but they do have those and maybe Coil too[?]) and start level grinding all the way to 60. However you DO need to start the 3.0 storyline missions to gain access to their respective guilds, but still you are capable of leveling the new job and doing the storyline/guild-quests simultaneously while progressing it.

We’ll be adding three new jobs: dark knight, astrologian, and machinist, and they’ll start from level 30. The job quests for each of these three new jobs will be located in Ishgard. If you want to start leveling these jobs as soon as you enter Ishgard, you’ll be able to do so without progressing through the new main scenario story. You’ll have to be at least level 50 for the new main scenario. By accepting the first job quest, you’ll be able to level up along with the story.

http://forum.square-enix.com/ffxiv/threads/230605

tl;dr: It's not until you get your new job to 50 that you need to worry about content gated behind 2.xx content (which you'll be able to do with your newly leveled lvl50 DRK/MAC/AST)