This time it’s about forging a new ring of power, as well as conquering fortresses in “massive battles” and dominating Mordor with your personal orc army (via NeoGAF). The successor is therefore called Middle-earth: Shadow of War and again includes the Nemesis system known from the first part. Original message (): A sequel to Middle-earth: Shadow of Mordor has not yet been officially announced, but the US store Target has accidentally leaked the first details.
Here you can find all the details, the release date and the first trailer for Middle-earth: Shadow of War.
Update (): Warrner has now officially announced the sequel. Shadow of War: Middle-earth sequel details: Shadow of Mordor leaked