I see it as a bit of a triangle because of how the exotic swords work against against the same/alternate 'elements'. Kinetic is one damage type that could be fleshed out, but I believe it would be seriously week against shielded PvE enemies. Solar, Arc, Void... How about: Toxin? Precedent being the damage from Thorn and the damage from Khepri's Sting. Just an idea...