Just another radom thought: How about guild leveling? Depending on the requirements you meet (time, members, etc... as mentioned above), types of people in the guild (skill-wise... mages, fighters, thieves, etc.), and guild quests completes, your guild could get a succesive title of sorts (Clan, Grand Coven, Band of Thieves, etc.). I doubt any other benefits could be attached to this (for balance), but the \"bragging rights\" would be a nice incentive.