
Well, it's been officially confirmed by Kevin Feige himself, although we all know it was true by all the so called past rumors floating around, he spoke to Buzzfeed stating that the Avengers roster WILL be changed by the end of Avengers: Age of Ultron.
In a statement comparing the Avengers films to Marvel crossover event comics Feige has said:
“The Avengers films, ideally, in the grand plan are always big, giant linchpins. It’s like as it was in publishing, when each of the characters would go on their own adventures and then occasionally team up for a big, 12-issue mega-event. Then they would go back into their own comics, and be changed from whatever that event was. I envision the same thing occurring after this movie, because the [Avengers] roster is altered by the finale of this film.”
Source: ComicBook.com