I highly recommend to really spur things up and surprise players (and to stop me from falling asleep) to add a purple-named "Big" Boss type to spawn once enough kills on a instance has been earned. It could be an extra large mummy or "giant" lich (similar to the SoS final boss) but it should be similar to a raid boss in power and be a challenge to players. Rewards could be a guaranteed ethereal key or motes and even a 1% chance for a spectral scale drop.