Uhh actually in a lot of Final Fantasy games they use Claymore/Zweihander type weapons. Think FF7, and to be honest I don't think Dragoon was anywhere intended to be the main dps... I mean... Monks have higher damage potential but I won't get into that. Fact of the matter is making the animations, skillsets, particle effects, weapon design, and adjusting the armors to avoid clipping takes a lot of time.

Classes like Berserker, and Dark Knight generally fit the bill of 2h sword, but who knows when we will see them due to popularity and the like. Highest priority is to fill in missing niche roles in parties, then secondary is popularity, and then finally whatever they feel like adding or bringing back from an old FF if they even wanted.

EDIT: Sorry since this was just mentioned, Garlean weapons have gunblades yes but their mechanics and fighting style are already in the game THUS making a class/job from that is a lot easier. ALSO I believe it was stated we would get a gunblade class sometime in the future... who knows when that is.