I'm in the camp that says Templars (essentially Holy Dragoons) will be the new Tank Job. I mean, we're going to Ishgard, Ishgard is a Theocracy, and a ton of their soldiers use spears. So from that standpoint it'd kinda make sense.
I wouldn't be surprised to see a Templar tank job eventually come out of the Lancer class.

That said, doing that won't be adding a new class, but a job on an existing one, and it was stated that a new tank class will be added to the expansion.