Avengers: Age of Ultron
From all the stories, comments, interviews and insider scoops I think this movie synopsis by Conor Lawrence of MoviePilot is the closest to reality any journalist has come to figuring out the story line. It's still speculation of course but I'm willing to bet on this being more or less the finished movie we will see in 2015.
Tony Stark has built Ultron, a robot designed to protect the innocent while the Avengers relax. While Stark is busy cohorting with the other Avengers, Baron Von Strucker gets to Ultron, and zaps him with Loki's Tesseract staff, possessing him. Ultron turns against the Avengers, and they flee Stark Tower before he can wound them. He ends up wandering around, and destroying entire cities in his wake. The Avengers gather as many allies as they can; the Falcon, War Machine, and two young supers named Quicksilver and Scarlet Witch. Ultron continues attacking major areas, and decides he needs a legion of his own, like Stark's Iron Legion. He steals Stark's, modifies them, and builds others. He makes one of them intelligent, and names it the Vision. He then decides his armor isn't strong enough, so he travels to the African nation Wakanda, where he steals mass quantities of vibranium to upgrade himself. He attacks the kingdom of T'Challa, the Black Panther in order to retrieve the metal. The Avengers begin to fight amongst themselves, partly due to Bruce Banner's temper. Banner hulks out, forcing Stark to don the Hulkbuster armor and fight Bruce. After the fight, Bruce gets incapacitated and put in a rocket, which gets set for space. The remaining Avengers then unite with T'Challa, and face off against Ultron, and finally defeat him, then recruit Black Panther and the Vision.
Mid Credits: Strucker laments over his failure with Ultron to a figure offscreen. The figure then angrily condescends him for his failure, and comes out of the shadows, revealing himself to be the Red Skull.
Post Credits: Thanos watches the events unfolding between the Avengers and Ultron, smiling at how easily they fall before the robot.
Written By Conor Lawrence from Movie Pilot