Many people are speculating that Tony Stark or Loki's scepter will lead to the creation of Ultron. I however, have a different theory on the creation of the Avengers arch-nemesis.
In Captain America: The Winter Soldier, Arnim Zola created an algorithm that was designed to eliminate potential threats to HYDRA. Even though Captain stopped HYDRA from killing 20 million people, they won't stop until their task is complete.
In secret HYDRA is creating the ultimate weapon: Ultron. He will be programmed with his algorithm to eliminate the threats. After becoming sentient, he realizes that he is just doing HYDRA's dirty work and turns on them. Not wanting to serve humans who he views as lesser beings decides to kill all of them.
I do not believe that Ultron will be created by Loki's staff because in The Avengers, when Loki tries to take control of Tony Stark it doesn't work because of the arc reactor in his chest. This leads me to believe that the Mind Gem only works on living things.