Swapping weapons between classes as someone mentioned has been done before as knives used to be PLD weapons until ROG/NIN was added and they paired them for the new job.

As for what jobs could make use of an actual 2 handed rifle. Ranger from FFXI comes to mind as one of their proficient weapons was a 2 handed rifle while Corsair got the more pistol like hexagun. FFTA2 has Fusilier though the game portrays most gun animations as pistols. It also had a job class of sniper that specialized in greatbows but could probably be adjusted to use a rifle easily for FFXIV. Though I think in terms of firearms from FFTA2 I'd probably be more interested in the 2 jobs using hand cannons Flintlock and Cannoneer.