So the Avengers Assemble sequel is called "Age of Ultron"! Here's some of the info regarding the Avengers panel held at SDCC:
Marvel caught up with Joss Whedon (what follows is my own transcription of their conversation as it has yet to be released officially after airing just moments ago on their live blog) at the San Diego Comic-Con to talk all things The Avengers: Age of Ultron. Surprisingly, the writer and director revealed a lot more than any of us may have expected! After saying that he always wanted to use Ultron as the villain in a sequel, Whedon would go on to clear up some misconceptions that the title has caused. "Well, because there was a book called 'Age of Ultron' quite recently, a lot of people have assumed that is what we're doing, but that is not the case. We're doing our own version of the origin story for Ultron. In the origin story, there was Hank Pym, so a lot of people assumed that he will be in the mix. He's not. We're basically taking the things from the comics for the movies that we need and can use. A lot of stuff has to fall by the wayside."
So, if Hank Pym doesn't create Ultron, who does? Well, Tony Stark seems to be the most obvious candidate, especially after Iron Man 3 showed that he had managed to create sentient versions of his armour. "We're crafting our own version of it where his origin comes more directly from The Avengers we already know about. It's a little bit darker than the other film because Ultron is in the house. There's a science fiction theme that wasn't there in the other one. Ultron is definitely something that evolves, so we're going to get together a couple of different iterations. Nothing can be translated exactly as it was from the comics; particularly Ultron." Finally, he was asked about the scale of the movie and which character he is most enjoying writing and revealed: "It's very much a global Avengers film. A lot of the movie has to do with their place not just in America, but the world. Part of the fun for me, definitely this time around, is writing Hawkeye. He did get possessed pretty early by a bad guy and had to walk around all scowly for most of the movie so now it's nice to actually have the character there and see him interact with the other guys."