It's just an idea, but how about introducing Guild Headquarters in the game. It would be a place where players of any level can join the guild, then if they wish, for-take missions (as side quests), and earn themselves gold/exp/gems etc.
Here, missions can be made available by admins/mods (or whoever is chosen to run the guild), but at the same time players can make requests too. Say for instance, if I was after a certain item, I could post a request at the guild asking somebody to find it for me. If they complete the mission, that person would be rewarded at the Guild with something I submitted (gold/items/gems etc), relative to the difficulty of the mission.
There could be missions that involve killing a certain number of enemies, checking out a special location, or even maybe slaying other players. Limits could also be introduced to a select number of missions. The limits could range from player turns, to game days or even till you reach a specific level before the mission can be classed as complete. Missions will also vary in length according to what needs to be done.
Taking that in to account, players could gain ranks exclusive to being a member of the guild. These ranks would roughly identify how many quests a player has taken, and how well they get the job done. A player may also need to be a specific rank for certain missions. This could be due to the difficulty, or maybe because a player requested a higher rank character for the job.
I think it could work quite well, and may increase relations between players. There is a lot more that can be introduced to this Guild idea, though it depends on what people thing about it so far.