This brought to mind yet another idea: A consumable or doodad from Xur, call it Elemental Modulator, which you consume to make your primary match the elemental type of your [i]currently selected[/i] subclass. The burn lasts as long as you have the subclass active; it expires when you switch subclasses. If you're the kind of person who runs pretty much Subclass X, then your primary will always have that burn. If you change it up, you need to use another Elemental Modulator to reattune your gun.