Okay, so in Entertainment Weekly's new Iron Man 3 cover story, they drop this little bombshell:
A source confirms to EW that the twin characters Quicksilver and Scarlet Witch will be joining The Avengers 2, which Whedon hinted at recently when he said he was writing a brother-sister duo into the movie. Feige maintains a poker face on that scoop, saying only: "I'm not confirming or denying. The draft could change six months from now."
If Joss Whedon got approval to have them in there now, I don't think he's dropping them later. Here's why: these particular characters are hugely significant in that they belong to both the Avengers and X-Men universes. It has been mentioned before that they're the only two characters Disney/Marvel and Fox both have the rights to use - seeing as how they're Magneto's kids.
Look at what happened in the first Avengers movie: we were all so sure the villains would be Skrulls, even as the nit-pickers among us pointed out that the Skrulls belong with the Fantastic Four rights at Fox. So instead, they were given a whole new look and called Chitauri, the alternate-universe versions of the Skrulls, and did not evince any shape-shifting abilities. Had Whedon needed mutants with the specific powers of Scarlet Witch and Quicksilver, he could easily have made up rip-off versions with different names to placate Fox out of any legal action.
So why would Whedon use these particular characters if he were hamstrung as far as using huge chunks of their origin story? Well, could there be a bigger post-credits fanboy joygasm in Avengers 2 than Ian McKellen stepping out of the shadows and going, "Hello, children!"?
I'm not saying that'll happen. But I think this indicates that high-ups want it to happen.