They could just add a new class or job down the line that does the healing/buffing through ammo gimmick. We aren't staying a 7 classes/7 jobs forever. The mechanics of how it would work would be best on something new rather than shoehorned on any existing class.

FFTA-2 had the Flintlock job which healed/buffed through ammo, and I could see something like that appearing eventually as we need multiple healing classes.