Add a Nethack-like component as a randomly generated adventure location on the map.
Nethack and Rogue-like games are awesomely addictive, although they are just ASCII (or, in certain cases, ANSI) graphics. The addictiveness, other than heavy RPG-like customisable elements, probably stems from the randomness factor of the game.
How about making a 'maze' or something similar, which would be randomly generated, which players could use to entertain themselves when they've completed all the prior quests (or, failing that, to help themselves level up so they can achieve previously unachievable quests)?
There they can find random objects, fight with monsters of their level (or the level of the party), gain levels, and so on.
The randomness element would ensure that it never gets boring!
Caroline Seawright commented
Sounds great! ^_^
AdminDerek (Manager, 10x10 Room) commented
We have something like this in mind for what we're calling Exploration Mode, which is one of the stretch goals of our Kickstarter. Players would choose a region from the map to explore, and the game would generate a mix of challenges in one or more locations within that region. While not as story-driven as our main quests, these would still lead to an ultimate goal rather than being disconnected.