If you kill somebody with gold sub, you should get golden skull. Silver with silver sub, bronze with bronze etc.
Amount of skulls would be limited with some cooldown (like one every few months) and skull is only given in the event that the character with highest LP in the account dies.
They could work kind of like normal skulls do, but maybe give a bit more LP.
This would add more value to the subscriptions and excitement to people who kill others!!