Why not create a new exotic weapon that can be equipped as either a primary secondary or heavy weapon. It would have the essence of the Husk or Eidolon Ally, because it can literally morph itself (using interchangeable parts). It could start out like a mini sub machine gun, but as you add perks and parts it can move into another class. Add a barrel extension and front grip and you got a primary weapon. Add a grenade launcher and you have a heavy weapon that could do multiple things (fire, launch).