In an interview with IGN, President of Marvel Studios Kevin Feige spoke about some of characters being introduced into the Marvel Cinematic Universe and the possible shifting roster of Avengers.
Regarding Doctor Strange, Feige said that we would get some casting news for the good doctor "realtively quickly". He cautioned that "I don't know if it'll be as quick as next Saturday" … "But it'll be relatively quickly". Saturday of course, refers to San Diego Comic Con 2014.
He also commented on solo character movies and Strange appearing in Avengers:
"It does make sense [to have standalone franchises], but I don't think it makes sense for this character, who has clearly woven in and out of that stuff [The Avengers] in the comics" … " But if there's a character like that who clearly interweaves over the course of many stories, I find no reason to go, 'We're not going to do that in the movies. We're going to keep him in his own movies.' So there's always the potential for it [Strange to join The Avengers]."
He feels a similar sentiment for another pair of characters, those of course we know will be appearing in the Avengers: Age of Ultron:
"That's one of the reasons why Wanda and Pietro [Quicksilver and Scarlet Witch] have shown up in the new Avengers film, because we like the notion of a shifting roster, of a growing roster."
So it looks like we may get an "old order changeth" somewhere down the line (those actor contracts are pricey!). As Feige says, "Go into the comic bin and pull up any Avengers issue from 1960-whatever and 1980-whatever, and they probably don't match. I think that's fun."
Quicksilver and Scarlet Witch (and maybe Doctor Strange?) will be appearing in Avengers: Age of Ultron May 1, 2015.