There doesn't seem to be any clear pattern for job introductions and expansions. So i can't say if there'd be a tank next expansion or even the one after. But I don't think there could be a better idea for a tank than something akin to Noctis. And quite frankly, the idea already fits excellently. Overall, i think this 'Kingsglaive' class as i'll call it, would be a good middle ground between Gunbreaker and something more defensive like Paladin, having a more active rotation but being a little more defensive.
For one, Noctis and his arms already would serve all the typical roles of a Tank class.
A warp strike would serve as a good Charge attack like Plunge or Intervention. This would increase in potency the further away you are, yet it would have the longer cooldown to deal with, AND would cost MP on top.
The Kings' arms animation would work PERFECTLY for mitigation. Mitigation would probably be less reactionary and more proactive like Paladin or Dark Knight, and less like Gunbreaker or Warrior [I don't entirely know how Warrior works so that statement may be wrong, all I've heard is that it's a more fierce class which makes me think more reactive stuff would fit better].
Meanwhile, a Magic Ball or a gun would serve for a perfect ranged pull. Greatsword would work for AOEs, while the main sword would work for the main combos, with the daggers or spear coming out to play. 'But that's multiple weapons,' i hear you ask. Simple. You would probably equip a Ring or a basic Sword as the weapon, but still being able to use all those weapons- perhaps it could be explained as a sort of aetheric manipulation used to shapeshift the sword.
This could be done a couple ways- either by having a few endings to the combo, or having a combo not unlike Gunbreaker's second combo. Perhaps a basic combo could be introduced, plus a second combo, beginning with the Sword, and allowing you to use an 'Arms Swap' skill for the following two moves, allowing you to use either the Sword, Spear, Daggers or Greatsword for the rest of the combo.
Obviously, the Class 'Gauge' would be designed for showing you the weapon you have equipped [Out of the four available].
What do you all think of this idea? The possibility of this getting in is absolutely zero to nothing, but it was a really cool idea i thought of and wanted to share with you all. Though, i suppose Gunbreaker did get in solely as being an homage to FFXIII so... maybe?~