In my personally opinion, the jobs should start at level 1, which also should have been the case for the Heavensward jobs. However, in terms of what will probably happen, it's safe to expect that the new jobs will start at level 30 since that is what Heavensward has established.

If anyone is wondering why I think that the jobs should start at level 1 as opposed to level 30 because "that's when all jobs start" it is for a very simple reason. Speaking in the context of pre-Heavensward, it is indeed true that jobs began at level 30, but it was also true that when beginning your training with a new weapon type (or skill set) you did so from level 1. Heavensward broke that rule, letting players jump straight to level 30 with an unfamiliar weapon. If that rule is one that can be broken then so can the rule about jobs starting at level 30.