I agree that Tom Sawyer should be among the classics. They already did two Oliver Twist films; Oliver and Company and a made-for-television film in 1997. Some of Shakespeare's plays such as "Measure for Measure" could also be a good choice. Also Charles Dickens' Great Expectations, the Count of Monte Cristo by Alexander Dumas, Illiad, Odessey or some of Hans Christian Anderson's other stories they haven't used such as the Travelling Companion, the Nightingale and the Wild Swans or Six Swans by the Brothers Grimm.