'Avengers: Age Of Ultron' was not just a title they created on the fly. Director/Writer Joss Whedon and Kevin Feige already had a clear vision on the franchise's next step. Casting is also given a lot of foresight-both addition and subtraction.

Marvel Studios Boss understands that an effective super hero team needs to have balance. Also, the cast of the Avengers have taken giant steps career wise, in part because of the immense success of the film. Marvel had a taste of that growth when contract negotiations for the sequel dragged on, and it's not just about the money.

The highly publicized disappointment of Jeremy Renner, who played Hawkeye in the film, was the best example. The actor can now headline his own film, thus it's not easy for him to be a fourth wheel.

Kevin Feige discusses these issues on SFX magazine:

"We didn't want to merely have a '2′ after Avengers so we wanted to give it a subtitle. We have various things that we were looking at and Age of Ultron - which just happened to be a the name of a comic book series as well - was our favorite. I first met James Spader two or three years ago. I thought it would be great to have him in our universe and we finally found the right role with Joss's version of Ultron."

"It's always a balance with any movie, not to overwhelm it with too many story elements. Certainly when it comes to comic book movies, you can fall into the trap of having too many villains and in the case of team-based movies, having too many heroes. But what Joss has in mind to bring those characters into the story happens in a very natural fashion over the course of the story. It's also the tradition of the Avengers to switch up the roster every now and again and to have new characters coming in. It's just part of the fun, but we wouldn't have included them if it had just been a case of 'Oh, we want two new figures, two new costumes.' It very much flows with the story that Joss is creating."

We can see that the 'Avengers' sequel is in good hands, but there is also a hint of change, something Joss Whedon has already teased. While speculations continue on who are the next additions to the Avengers cast (Ant Man, Black Panther), it should be noted that balance means there would be subtractions.

The excerpt from Whedon's Entertainment Weekly interview:  "But I'd have to have a really good reason, a really great sequence for [Marvel execs] to go, 'We'll cut off a potential franchise, that's fine!' They know as any good studio does, that without some stakes, some real danger, how involved can we get? We don't just rule it out across the board, but neither is the mission statement 'Who can we kill?' We try to build the story organically and go, 'How hard can we make it on these people?' You go to the movies to see people you love suffer-that's why you go to the movies."

The phrase 'potential franchise' sticks out. It may not be a death, but a shift in direction. It is possible that as the Avengers go cosmic, some of the heroes may not be up for the change. Captain America seems to be building their own 'universe' on the ground, and heroes like the Black Widow and/or Hawkeye may stay with him, while the rest of the Avengers go cosmic. This is, of course, a theory at best.