Well my idea, is an opportunity to earn a skill as a side-quest. These side-quests are free and can only be played after you've completed the entire quest for free: Cooking, Crafting, Firemaking, Fishing, Magic Mining, Prayer Rangeing, Runecrafting and Smithing Woodcutting. These side-quests available to members can only be completed if you reach a specific quest level (this depends on the Runescape Staff).
Example of side-quests: You finnished all of your quests, but you have a very low level of prayers, and you have the most difficult time obtain giant bones because of other players. At the Monastery you will find one monk who will aid you.