The Knights of the Nine add on, adds nearly 10 hours of game play with a great mini plot line that as quoted from IGN "lets players join an all-new faction, an order of holy knights, and lead them into battle against a sorcerer-king while exploring Cyrodill searching for relics which will answer many questions surrounding the Ayleid ruins. With the holy armor and weapons of the Divine Crusader players can restore the Knights of the Nine to their former glory."

The Shivering Isles expansion has been scheduled for spring 2007 and should add a good 25+ hours to the already 100+ hours of game play. The story line at the moment is not final, though it will most probably involve The realm of Madness which is the daedric realm ruled by prince Sheogorath. So I assume that you are going to have to do some heroic thing and stop him from doing some dastardly deed :P