User blog comment:Gcheung28/Fannotate the Latest Avengers: Age of Ultron Trailer/@comment-24895270-20141025204554

Entirely speculation on my part:

Ultron was a dormant AI created some time ago possibly by a young Hank Pym and Howard Stark. Variety had reported that Hank Pym would be the villain in Ant-Man so maybe Ultron is evil on purpose? Possibly with old ties to Hydra?

Based on the end of Iron Man 3 and the trailer, Tony builds an Avengers robot police force. It might be used to stop whatever protest or riot is happening in eastern Europe. Maybe Ultron takes over and massacres everyone leaving the Avengers looking like killers to the public, including Scarlet Witch and Quicksilver who join up with Ultron.

Hydra member Klaw discovers vibranium in wakanda. Ultron gets all Ultron Primed and becomes final boss mode. SW and Q come to their senses and join Avengers for big fight (maybe with Black Panther too?). Defeat Ultron. Half team leaves. Caps Kooky Quartet ensues.