I'd like to see someone who's action skill is actually a custom gun, maybe the same company that makes Axton's turret. Have a tree for elemental, explosive rockets, and straight firerate/damage. His gun has unlimited ammo (maybe). Since essentially all these skills would affect how the gun fires, it could be the closest thing we ever get to make your own gun.
Screw it make a space cowboy whos special ability is to duel wield 2 unique revolvers which can be upgraded to have melee capabilities elemental capabilities or straight damage /explosions/fire rate abilities.
Just kidding, kinda. I really have no idea what im talking about.