Along with a full list of achievements, Middle-earth: Shadow of Mordor opened the ol' evil eye on the upcoming season pass. The pass's exclusive content includes the "Guardians of the Flaming Eye" Orc Warband mission where you'll face Sauron's elite in front of the Black Gate. Plus you'll get a head start accessing the "Trials of War" challenge series, pitting you in a series of top-the-leaderboard challenges against friends and strangers alike.
The season pass throws in a couple more story missions and "access" to future content, whatever that means. I'm almost certain that future content will likewise be available to other players, so.
Middle-earth: Shadow of Mordor hits the PlayStations and the Xboxes on September 30, and PC on October 2.