My understanding of Lucas' original concept when he started writing was that his story outline started with Obi-Wan and Anakin, continued on through the events of the OT and then continued even beyond that. It wasn't until he started fleshing things out into script form that he realized just how much potential material he had and eventually worked his way back down to the "Star Wars" we now know. So in theory, he's got some leftover notes vaguely spelling out what could happen in a third trilogy.