Of all the Marvel films, the one that is arguably taking the longest to get to screen is "Ant-Man". Filmmaker Edgar Wright was attached and working on the project back when the first "Iron Man" film went into production, but Wright's other projects took precedence.
Now, with production finally about to begin on "Ant-Man," Marvel Studios President Kevin Feige tells Empire (via Latino Review) that having the story locked down for "Ant-Man" all those years ago has impacted how Marvel has built and established its cinematic universe:
"We changed, frankly some of the MCU to accommodate this version of Ant-Man. Knowing what we wanted to do with Edgar and with Ant-Man, going years and years back, helped to dictate what we did with the roster for Avengers the first time. It was a bit of both in terms of his idea for the Ant-Man story influencing the birth of the MCU in the early films leading up to Avengers."
Feige goes on to say that's why the character of Hank Pym wasn't in "The Avengers" or the upcoming sequel - essentially because it wasn't necessary:
"It was two things – a story Edgar wanted to tell that we’ll see with Michael Douglas and Paul Rudd, and now we’re 11 films in, it’s looking at the characters we’ve already introduced who already have the technology and the brains to do... certain things [Laughs].
Feige also teased the appearance of Thanos in "Guardians of the Galaxy" and confirms that they do have an actor set, but he's "not sure we want to announce it yet."