While I do love the Gunblade itself, I'd go more towards Solider being the class, and give them a self healing ability to self and nearby party members is is based on a timer as a last resort to healing.

Then make the Gunblade a usable weapon in the mix, while equipped you cannot equip items to ranged or ammo. It will have a short distance, in order to separate RNG and COR from them, possible 5 yalms or so.

Similar to ninjitsu, the ammo is "spell" based however requires the ammunition to be in the inventory. Soilders can only equip chain or light armor no plate (and definitely no subligar.) making them a damage dealing class however would not be able to tank as hard as WAR. I like the ammo's added effects that you all are talking about, however they are able to use all 1 handed weapons excluding rapiers. No native dual-weild trait, cannot equip shields.

I think one thing we're forgetting about is not just the extra "job" to throw in, but the content that would come with it. Story, skills, quests, ect.