I got a suggestion for a dungeon. 10 bosses with strong health and damage that drops set pieces for each class , the set should have some set bonuses, the bonuses is up to you Admins to decide if it should be very OP or not. But i think that it could be fun to have 1 dungeon like that.
Also a quest chain that require serious amount of hours to reach a full and very good custom armor set. Upgrade the gear like 3-5 times.