I have this idea, maybe it exists or is in the works, but it would be like an artifact mod that would cost like say 6 slots or more or an exotic perk that would match elemental shield types.
If an exotic perk then its trait would read something like - "Aiming down sights at a visible elemental shield for a period of time and this weapon will briefly match that elemental shield type." You would know if it proc'd by some sort of glow or sound like unstop.
If for a artifact mod then - "Aiming down sights with a (plug in weapon type) at a visible elemental shield for a period of time and this weapon will briefly match that elemental shield type." If the weapon already has an elemental type then it would resume that type when say aiming at red bars for when trying to do a bounty that requires say Solar Kills.
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So... Hard Light and Borealis.