From http://blog.us.playstation.com/2015/...d-new-details/ on Mar 07, 2015.
I'm surprised this hasn't been a more talked about subject. Based on the clarified answer to the above question, it would seem that it will not be possible to play any of the three new jobs unless all of the 2.0 story is complete. If that is the case it strikes me as an odd decision considering any new players drawn by the expansion will be locked out from playing any of these jobs until they level a different job to 50 (contradicting the supposed "level 30" requirement that was stated in a Massively article) and complete every quest in the main scenario. Any existing players who want to level a second character would also need to fulfill those same requirements.Will we have to finish the main campaign or reach a certain level to access Heavensward's main content?
Editor’s Note: After publishing this story, we received clarification from Square Enix for this question. The updated answer follows, but we’ve left the original answer in the story for transparency’s sake.
Naoki Yoshida: You will have to already own A Realm Reborn, plus complete all of the main story quests up through Patch 2.55. After that, an introductory quest for Heavensward will become available; accepting that quest will then guide you through the gates of Ishgard. All of the job quests related to the new jobs will be in Ishgard as well, so the condition to access those will be the same. If a player had stopped playing around Patch 2.2 or 2.3, it would be best to revisit the A Realm Reborn main story quests to get back into the groove as you head towards Heavensward!
Original Answer: If you already own A Realm Reborn and you get Heavensward, you’ll be able to at least enter Ishgard. The new jobs added in Heavensward are not tied to the main story, so those are accessible as well. In terms of the story, if you want to enjoy the Heavensward story arc, since it’s tied to the upcoming 2.55 patch, it’s a prerequisite to have finished that content. If a player had stopped playing around patch 2.2 or 2.3, I’d recommend choosing one of the new jobs and leveling that up while catching up on the story, then transitioning into Heavensward.
Even if people were to say that it's "easy and doesn't take that long" I still don't understand why the new jobs would need to be gated like this. The Dragoon story has ties to Ishard, yet we don't have to meet such steep requirements to play that job. I don't see why these new jobs should be treated any different. If by any chance the information provided in the above article is incorrect or misinterpreted I would like to know.
To add a little clarification to keep things on track, this isn't about needing 2.0 story to access Heavensward areas. I'm okay with that. What bothers me is that SE felt a need to lock the new jobs behind that gate as well when the job quests could have been made available in existing zones (still requiring a Heavenward purchase to access those quests, of course) thus making them more accessible to new players.