Just like NIN wasn't gated behind the Yugiri plotline, I'm willing to bet none of the new 3 jobs will be really gated behind 2.55 -- perhaps some quests for them, but not acquiring the job. I think it telling that nowhere in the descriptions of the new jobs does it ever mention the main city of Ishgard, and DRK are 'pariahs in their own lands' -- a non-Ishgard base seems most likely for them. AST could easily be outside Ishgard as people from Sharlayan could, like Yugiri, leave their homes to impart this job. Even Machinist could have a building/smelting/creating Skysteel manufactory outside Ishgard (less pollution, too). I think SE wants these new jobs as a selling point to draw in new players - good business sense // marketing them as a major component then locking them behind the main story isn't newbie friendly -- and we saw from NIN that SE is very open to glossing over plot in order to fulfill a customer desire.