Spoilers lie ahead for Avengers: Age of Ultron.
So Ultron was one the more well-received villains of MCU, but is it possible that he'll return in phase four? I believe it's highly probable.
In the comics Ultron is known for coming back in better body each time, this was briefly touched upon in the movie, but we clearly saw all of the Ultron drones perish, right? Probably not. If you remember his plan, bringing the small nation of Sokovia into space and turning it into a cataclysmic asteroid, he planned to use his new Vibranium body to survive that, but we saw it pretty easily torn apart by lightning and repulsor. This brings up the question of how would it survive being on a giant chunk of rock being hurled at thousands of miles per hour to crash onto a larger chunk of Earth? Elementary: another body.
He could easily have another body on a hacked satellite that he would upload himself onto to once he caused humanity's destruction. It makes perfect sense, and could bring us more Ultron.