I personally would like Gun Class to take a page out of Star Wars: The Old Republic's book and become Ranged Healing & DPS.

Right now Bard is Ranged DPS & Support, Gunner could potentially be a new form of Healer.
Focusing on Magi-tech instead of Magic, which from a Healer standpoint would be a breath of fresh air.

Honestly FFXIV has too many DPS as it is. I would like to see Gunner turn into something that would help balance out the class archtypes.
I remember reading that Ranger is in development a few months ago.
So Gunner being DPS would be a clone of that, to have Archer turn into Gunner would alienate the Lore of Archer imo.

Taking from Lightning & having a Gunblade would give them a unique weapon.
Give the class Stances/Modes and they can cycle between close and range.
Which could give the class some more customization.

I'm just spewing out ideas. I have wanted this class since FFXIV ARR's BETA.