The Limsa leader who's name I can never spell right uses 2 guns. Also one of the quest NPCs for MCH uses a 1 handed axe + a gun. Raubahn dual wielded 1 handed swords until he sort of ended up only having 1 hand.

There are a number of other weapons in the FF franchise of games that haven't been put to use here yet

I wouldn't mind more classes making use of shields. Gunbreaker could have been made to make use of a shield given the size of the weapon though it would have detracted a little from that FF8 squall image. One of the tank characters in the mobile game Kings Raid actually has this setup as a weapon. Given that a shield is a general defensive implement I just don't see why its really been limited to Gladiator/Paladin. They have for the most part phased shields out of Conjurer/WhiteMage Thaumaturge/BlackMage as they'd also pretty much phased out 1 handed wands/rods for them as well. At the same time that offhand/subweapon slot sits there empty for every other combat class.