For example, "get unique item X from top 10 player Y". It would be interesting if the top players had to deal with hordes of lower players after their loot.
This could be turned into an interesting mechanic. Instead of making it the top 10 players, you could assign a number of GM's to act as semi-pc's. Their characters would react as an NPC when a player clicks on them, and they can modify or rotate the quests they give. At the same time, they move around and level with the players. You could have some of them at the max level, and others assigned to restart, level up new characters so there are always these semi-pc's at many of the levels.
Furthermore, you can tie storyline/faction warfare/whatever into this. Make helping the semi-pc's level up, or just helping in general get buffs/bonuses for whatever team helped them.