Total Film brings us the following from Whedon on why Ultron will be the next villain:
“I was pitching Ultron before I took the job on the first movie. I was like, ‘I don’t know if I want to do this, but for the second one, you should totally do Ultron.’ Because he’s been a thorn in their side as much as any other character over the years and particularly back in my history.
“He was great. He’s somebody who can juice things up and he’s a real problem for the Avengers. That’s not always easy to find. It’s not like the Vulture is really going to give them a hard time for 20 minutes.”