I do think that a gunblade-wielding job is a very real possibility; one that is fast approaching. If memory serves, when the subject of a gunblade job came up years ago, the response alluded to the chance of something like being more likely when we reach Garlemald. At this point in the story we are essentially at the Empire's doorstep and I have a hard time imagining them stopping the momentum, but the likelihood of pressing forward will become more apparent with subsequent patches that develop the story.
The interesting thing about a gunblade job is that, unlikely nearly every other request, it isn't based on a classic identity, but rather an iconic weapon. SE has voiced an interest in presenting jobs where they are allowed more creative freedom. In the case of a gunblade job, they can take it in whatever direction they want.
With that said, in my eyes I can easily envision a gunblade job as a tank. My justification is that, since the gunblade has been established as specialized imperial weapon, what else is there related to the Empire that can be considered iconic? How about their heavily armored high-ranking officers? If not actual imperial armor, I can at least imagine our gunblade job being donned in an imperial-esque armor style. Also, I would like to note that, rather than being trained by an Eorzean citizen who happened to pick up a gunblade and figured out how to use it, I personally love the idea of having the Warrior of Light trained by a former imperial officer who has ceased to follow the rule of the emperor.
As a lore-related side note, although the Eorzean Alliance has expanded, the Empire is still very powerful. I have a hard time imagining them being defeated through brute force. Perhaps a future plot point will revolve around trying to persuade imperial citizens to rally against their emperor, potentially escalating into a civil war. The gunblade trainer mentioned above could be connected with such a movement.