Adding Etheric Light to the loot tables in the existing raids would give people more incentive to go back and run them again. The drop rate wouldn't even have to be very high - there could be a 5% chance of it replacing an existing drop, whether from a boss point or chest (and a 20% chance at the confluxes because it's probably the hardest point of that entire raid and has no drops currently) and it would be enough to make it worth running again for anyone who already has the gear they want. I wouldn't mind keys, either, though I'm not sure if those would fit.