3. The Yuuzhan Vong Invasion
The New Jedi Order was a series of novels set 21 years after Luke and pals destroyed the second Death Star. It focused on the invasion of the New Republic by an extragalactic race known as the Yuuzhan Vong. These fierce aliens were devoid of the force, stopping the Jedi being able to use the force directly against them, who used living ships while being hell bent on destroying everything in the galaxy.
Why it might not make a great film: Firstly, the series was far darker than the movies and any previous entries in the expanded universe have generally been. The Yuuzhan Vong were directly responsible for killing over 300 trillion sentient beings and the destruction of dozens of planets by throwing moons into them. We know Palpatine had all of Jedi killed, but this is on a completely new level. The whole culture of the Yuuzhan Vong was centred on a philosophy of pain; they would purposely mutilate their bodies and cover themselves in tattoos to bring them closer to their Gods. Even Ewoks would not be able to brighten a movie involving these guys up. Fans generally didnt like the direction of the series and the death of many established characters. Despite the good guys eventually winning out the tone of the series was incredibly disheartening. The only way to get fans more upset would be if Jar Jar Binks made a surprise appearance. Oh and they killed off Chewbacca.