Avengers 2 Coming in 2015?

With the official statement from parent company Disney saying Joss Whedon's involvement with Marvel Studios is set to go through to June of 2015, there's been a lot of talk pegging the end date as the approximate release for the next "Avengers" sequel. But why?

For one thing, given the success of the first film, a sequel is inevitable. But primarily, it's because Whedon will need time to introduce more Avengers into the Marvel film-verse. After all, introducing Marvel characters and bringing them into the fold under S.H.I.E.L.D one by one worked for Whedon and Marvel before. The timeline worked like this:

- "Iron Man" set the stage by introducing Marvel movie fans to Nick Fury in a post-credit scene.
- When "Iron Man" producer Avi Arad brings in Robert Downey Jr's Tony Stark post-credits to the "Incredible Hulk," he establishes that the gamma bombed greenie and power suited Stark occupy the same universe, and would eventually work together.
- Whedon edits the post-credits scene in "Thor" to connect it with Fury, and brings the cosmic cube into the picture, which would feature in "Captain America."
- Serving as an uncredited consultant, Whedon made a few rewrites to the "Captain America" script so it would go beyond just a stand alone film, but to to tie it into the Marvel Cinematic Universe, in another post-credit scene. The film's subtitle, "The First Avenger" only served to whet fans' appetites.

While it was very "Rube Goldberg," it very clearly worked before, so why change the formula now? It seems as though the dominoes are already in place, as there are already plans for movies for future Avengers members movies, including Black Panther, Ant-Man, and also speculation concerning other potential future members like Wasp, Dr. Strange, and others. And what's more, Marvel has already announced that the OG Avengers have sequels in the works, some even already in production, like "Iron Man 3." 

The "original" Avengers members' movie sequels will debut according to the dates below:

"Iron Man 3" - May 3, 2013 
"Thor: The Dark World" - November 8, 2013 
"Captain America: The Winter Soldier" - April 4, 2014