I don't know if this has been done before by design, if so let me know how it worked out. I'd like to see a system where players can set up quests for less experienced players. Maybe as an alternative to the AH/bazaars for collecting crafting items, maybe just for the heck of it. The player fulfilling the quest would get some combination of items, gold, experience (all donated by the player who wrote the quest), and reputation.
I'm not sure what to give the higher level players as insentive, maybe a super version of reputation as more players do the quests? Just brainstorming here.