Yesterday, Joss Whedon made a very short appearance at Marvel's Comic-Con and revealed the title of the Avengers sequel! That title is Avengers: Age of Ultron! They had a pretty cool animated opening sequence introducing the sequel that teased Ultron's look. It looks like we have another epic Avengers movie coming our way!
Age of Ultron is originally a 10-issue limited series comic crossover storyline. It was published earlier this year, and it involves the return of Ultron and his conquest of the Earth. Here's a more detailed description of the story,
Months after his return, the artificial intelligence known as Ultron decided to accomplish his goal of destroying humankind by launching a massive and abrupt attack on Earth, taking over the whole planet, killing thousands of innocents and dozens of heroes and villains who stood in his way. Only a few of them are still alive, in a secret refuge under Central Park, ready to reclaim freedom or die trying.Marvel doesn't perfectly adhere to the comic storylines with their films, so I imagine this movie is going to go in a little different direction, but still keep the main theme of the story.