Make an elemental ammo synth that would provide Solar, Arc, or Void damage to any kinetic only Primary weapon. They could work like Ether Seeds or Resupply Codes providing the temporary benefit for 10 min, or they could provide the full amount of ammo as a synth only do so with buffed ammo. They could be made available through the Gunsmith, Xur, or make them available as bounty or chest rewards.
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Could be a good reward for completing things like raids, nightfalls, and prison of elder for the 2nd/3rd time in a week on the same character.