Thanks to the unbelievable success of 2012's superhero fun-fest The Avengers, director Joss Whedon has complete agency to go buckwild with his geekiness for the developing sequel. As such, the filmmaker took the opportunity of 2013's San Diego Comic-Con to announce the subtitle and ostensible storyline for the follow-up: come 2015, Marvel fans will be graced with Avengers: The Age of Ultron, per HuffPo.
Ultron is a Marvel villain developed by Hank Pym, a scientist and superhero better known as Ant-Man. The mechanical Ultron became self-aware and contentious after his development, rebelling against the professor and setting out to take over the world. The "Age of Ultron" arc involves a slew of Marvel names, including the Avengers team we saw featured in 2012, plus Quicksilver (already confirmed to be added to the sequel), Wolverine and Spider-Man... neither of whom are particularly likely additions, barring Hugh Jackman's recent profession of a desire for a mash-up movie.
Of course, it is likely that Whedon will stray somewhat from the existing material, perhaps shifting the Ultron focus toward his established heroes for the sake of the mainstream fan base. Still, Marvel lovers will be undoubtedly glad to hear about this expansion of the big screen universe. More classic villains, heroes, and mythologies. All good things.