4. Thanos
The Avengers have already got Thanos' attention, so I'm sure he will be watching with interest from the cosmos with regards to their future actions and their plight against Ultron might attract his attention even more so. The Mad Titan let out a wry smile when the Chitauri leader, The Other, said that to challenge Earth was to court death, so he is obviously keen to do exactly that. While he is not due to fight the Avengers head-on until the third movie of the franchise, he will be a more established character by the time Age of Ultron is released (following the events of next year's Guardians of the Galaxy movie), so a less mysterious and more prominent role in this movie could be possible - perhaps working behind the scenes or from afar, guiding the newly-created Ultron in his quest for world domination.