Unless Ultron is powered by an Infinity Stone, or is indeed reverse engineered from the Destroyer armor, then I don't see what kind of stakes he really has aside from "robots killing all of humanity is sort of bad, especially since that includes Jane".
I'm thinking we're going to see him take a very general "Protector of Midgard" stance at the very beginning. He's adopted Earth as his home away from home, and he's going to want to defend it. That's it.
On the meta, his role may be sort of criticizing technology allowed to go rampant. He'll be a powerhouse and the outsider's point of view. But that's all I really see him contributing to the movie.