...and something like Blue Mage (also of the land, but instead learning abilities of enemies making it be DPS most likely with some support spells).

The ideas are of course endless and I'd still love for SE to implement and have us unlock ALL new classes with unique jobs, but the idea of having classes that are multifaceted like Arcanist I find to be very appealing and I hope it's something being considered.

The only downfall to this concept I considered would be when someone who already has a level 50 class unlocks a new job, they could be a main DPS/healer job and all of a sudden have access to a level 50 tank job, they may not know how to use it which could be disasterous in groups.

I think (read: hope) that because you learn job-specific abilities via job quests which rely on you understanding the fundamentals of a job and how it plays, otherwise you lose, that this would be negated and would be a non-issue, but it was still something to consider.