In an interview with Marvel's official video blog, The Watcher, Joss Whedon was asked "Will we be seeing Marvel characters who aren't yet in the preview photos? Are we going to be seeing other Marvel characters that we've seen in other films or other parts of the universe?" His answer?
No, really, who? There are already so many characters confirmed to appear in Avengers: Age of Ultron that the movie is fair bursting at the seams. We've got a full lineup of the old Avengers plus three new (Quicksilver, Scarlet Witch and the Vision), and many more of their supporting characters than we saw in The Avengers, including confirmed appearances from such temporally and cosmically far flung characters Haley Atwell (Peggy Carter), Tom Hiddleston (Loki) and Idrids Elba (Heimdall).
To be fair, we've also seen indication that the film will have some significant flashbacks or hallucinations in it, possibly related to the Scarlet Witch's established mind-control powers. That could be what Whedon is talking about. It's also possible that he's referring to some as-yet-unidentified but seemingly significant characters in the trailers, such as the woman pictured above, or the one unfamiliar face in that Lifting Thor's Hammer scene (see picture at left).
But it's important to note that Whedon didn't say this to any old interviewer. This was an official Marvel interview with Marvel staff: It's a deliberate tease. As such, it would be reasonable to assume that it's for a character of some significance to the future of the Marvel Cinematic Universe, and even that it refers to the first appearance of a superhero who's brand new to Marvel's Cinematic setting.
Highest on the list, in terms of likelihood, are Black Panther, Ant-Man and Doctor Strange, the only new heroes on Marvel's upcoming slate of movies to have been attached to a confirmed actor or actress. Chadwick Boseman's Black Panther is confirmed to be appearing in the third Captain America film, Captain America: Civil War, the second of Marvel's movies to follow Age of Ultron after Ant-Man (starring Paul Rudd), while Benedict Cumberbatch's Doctor Strange isn't due until the end of 2016. It's also possible that we might see the upcoming heroes of Marvel's Netflix offerings, Daredevil, Jessica Jones, and Luke Cage.
While it would be unusual for Marvel to show off a superhero design before announcing who would play them — which likely leaves Iron Fist (whose Netflix series does not have an airdate at present), Carol Danvers/Captain Marvel (her movie won't be released until late 2018) and any cast members from the Inhumans (mid-2019) out of the running — it's got one upcoming character for whom that might not be a particularly tricky thing to pull off.
After all, Spider-Man's costume covers his entire face. But given how recently Sony and Marvel buried the hatchet on rights to Spider-Man, that just might be wishful thinking.