I would like to see:
#1: Randomized world boss(es), that has a very low chance of dropping a unique item(s), also a critical kill for a class-specific quest that leads to a unique reward.
#2: Allow for rarity to be on display for optional wardrobes or mounts that have a limited drop rate from incredibly hard to find mobs - but, find a way to limit over-camping (perhaps, by limiting it to one per account).
#3: The items dropped by endgame bosses should be limited, but consistent with respect to the difficulty. Those specific pieces could immediately be used, however could be part of what is necessary to build an even stronger set of gear (ultimate endgame gear).
#4: Create a very strong and sought after class-specific quest line, that is rewarding as a result of milestones. It should take you across the entire expanse of the in-game universe, and should culminate with some of the hardest of tasks/kills to make it that much more rewarding. In the end, the reward should be on-par with endgame gear and contain a step-up quest line as new updates are introduced. Either a title, the look/color of the gear, or the effect on character should be visible to other players to show accomplishment.
Just some suggestions.