"Avengers: Age of Ultron" promises to be a humdinger and while the heroes to appear on the sequel have practically been laid out in the open, there could be more.
The only difference is that there could be some more heroes appearing but on a cameo capacity. And for now, two more names have been inserted into the fray – Dr. Strange and Captain Marvel.
Dr. Strange and Captain Marvel are two familiar Marvel heroes who were not necessarily in the limelight when you talk about the Avengers. Of the two, it is perhaps Dr. Strange who if quite far-fetched as he was never seen as an active member of the Avengers core.
Regardless, Dr. Stephen Vincent (Strange) is still a character that could appear. Though it may be of a limited capacity, do remember that the next Avenger movie is somehow prepping for future Marvel movies, particularly "Captain America 3: Civil War."
Even before Dr. Strange was mentioned, there were already rumors that Benedict Cumberbatch could appear in the movie. In one of the trailers, there was a scene where a gurney and surgical equipment were shown as well as the mysterious mansion located at a mountaintop. So is Dr. Strange included (in a cameo capacity) as well?
If so, it would not be surprising. Director Josh Whedon did mention that there would be several Marvel characters who would appear which were not shown in released photos. So it may be likely that Dr. Strange and Captain Marvel would be among those teased characters.
With regards to Captain Marvel, Whedon did mention that there would be four female heroes. If you count them, the only ones sure in mind are Black Widow, Scarlett Witch, and S.H.I.E.L.D. agent Maria Hill were mentioned.
So if you do the math, the fourth one could very well be Ms. Captain Marvel which could suffice once Ultron unleashes his wrath towards the end of the movie.