Just watched the trailer again and caught the shot of Loki's scepter from The Avengers, something I didn't notice before. Could it be what animates Ultron from Tony's scrapped suits?
Captain America: The Winter Soldier ended with a post-credit sequence of a hidden Hydra base where Baron von Strucker is in possession of the scepter. Judging from what the Strucker said, I believe Hydra is using the scepter to create super powers into individuals. A dangerous process since they mentioned dead volunteers and that only the twins survived the process. Since Tony Stark is still the one responsible for creating Ultron, it wouldn't feel right for him to just rush with the scepter (if they even allowed him to keep it) and use it to create Ultron. Way too much stuff to happen for what looks to be a two and a half hour film.