The Hobbit: The Battle of the Five Armies has been widely billed as Peter Jackson's last trip to Middle-Earth, but hardcore Tolkien fans have been holding out hope that one of the author's remaining books could form the basis of yet another movie set in the Lord of the Rings universe. What about The Silmarillion? Unfinished Tales? The Children of Hurin?
There may be a story worth filming in Tolkien's other Middle-Earth books, but to tell it, Jackson would have to overcome an entirely new challenge: the Tolkien estate, which holds the film rights to the author's remaining stories. "It's a legal thing," explained Jackson at a press conference. "The Tolkien estate owns the writings of Professor Tolkien. The Hobbit and The Lord of the Rings were sold by Professor Tolkien in the late '60s… the film rights. But they are the only two works of his that been sold. So without the cooperation of the Tolkien estate there can't be more films."
There's always a chance that the Tolkien estate could be swayed into selling the rights, but it would take a lot; after the success of both the Lord of the Rings and Hobbit movies, they've become awfully precious.