Here's what I think is gonna happen based on posts around here:
Movie starts off with Fury sending the Avengers to Von Struckers base, to steal Loki's sceptre, which they will eventually need to defeat Ultron.
They succeed and celebrate in the Avengers tower, where they are ambushed by Ultron and it quickly develops to a worldwide war against his minions.
By the end Cap splits from the team and creates his own little team with members like Black Widow, Hawkeye and Falcon. While War Machine dies at the party scene, and Klaue is just on the field selling technology to everyone. While Thor discovers what's going on in Asgard by the help of Heimdall, and Hulk gets shot out in space.
The end.
Just my speculation. Pretty basic.