If this really is the penultimate appearance of Tony Stark in the MCU, then it makes sense the character is front-and-centre in Age Of Ultron. Iron Man's the character that kicked off the Marvel Cinematic Universe in the first place, and in this second Avengers film, he has a glimpse of its potential demise as well. Whether the vision given to him by the Scarlet Witch was a premonition or simply a dream remains to be seen. What is for certain is that the wormhole through which the Chitauri jumped to Earth and began their full-scaled invasion of New York with the help of Loki is still active. Or at least, Tony says so, referring to the ever-present threat as part of his justification for building Ultron. He wants to provide a suit of armour around the world. Captain America argues that they're prepared for any Earthly threat but, Stark says, what about when they come from the stars? Tony's vision and ensuing concern suggest that the Chitauri are still very much a part of the MCU, and Avengers wasn't nearly the last we've seen of them. Given Thanos's reputation as a cosmic tyrant who brings in any Tom, Dick or Harry as an accomplice, they might well be a part of Infinity War.