Star Wars. Granted, we have one now, but they can't seem to get away from the Clone Wars. I would have preferred a series following the further adventures of the gang after Episode IV. I believe the cartoon in the infamous Holiday Special was created as a pilot to test for a series. Not sure why it didn't go through.
Also, an Indiana Jones animated series. Lucas once stated it was a "possibility" for the future, and I doubt we'll ever actually see one, but it would be cool. Though only if done old school animation and not CGI.