The Avengers has made over $1B worldwide for Paramount, Disney, and Marvel in less than three weeks. That's an incredible amount of cash, but with it also comes some serious pressure to keep the momentum going. Marvel has already confirmed that a sequel will happen(Duh), but with this first round of films out of the way, it's time to start laying the groundwork for the next phase.
One thing that I think we'll definitely see change in the sequel is the amount of female characters on the team. Black Widow(Scarlett Johansson) was wildly outnumbered in that superhero sausage factory of a squad, and with so many great female Avengers around you know that has to change. I mean, I'd like to see Cobie Smulders get more screen time as S.H.I.E.L.D. agent Maria Hill, but that's probably not going to happen. While talking to EW during a roundtable interview, Joss Whedon jokingly referred to the number of dudes on the team, while also hinting in his own way that will change.
Whedon: “I think we need to get some more men on the team,”
He said that jokingly before adding...
Whedon: “Studios will tell you: A woman cannot headline an action movie. After The Hunger Games they might stop telling you that a little bit. Whatever you think of the movie, it’s done a great service. And after The Avengers, I think it’s changing.”
As the creator of shows like Buffy the Vampire Slayer, Dollhouse, and Firefly, perhaps nobody does strong female characters better than Whedon. So it would make sense for him to want to add more to an Avengers sequel. The question is who would make the best fit?

The other two I see as candidates are She-Hulk and Captain Marvel aka Carol Danvers. She-Hulk makes sense because The Hulk turned out to be the most popular character in the whole movie, so why wouldn't Marvel want to double up their big green behemoths? Plus she comes packed with franchise potential. Captain Marvel is also one of the most important members of The Avengers, an ex-military brat with ties to the cosmic realm of the Marvel Universe. All indications point to The Avengers possibly dealing with some deep space threats, and Captain Marvel would make for an obvious addition. Some would say the Scarlet Witch would deserve to be on that list, but I believe she's most closely associated with the X-men line since she is the daughter of Magneto. So she may be unavailable as 20th Century Fox holds those rights. Then again, none of this will matter if Joss Whedon doesn't come back to direct. He's not signed up for sequels, so anything could happen.

Also, the ever chatty Kevin Feige was discussing the future of The Avengers line with Bloomberg, and he made clear that Marvel has five feature films in the works right now that aren't The Avengers 2. Three of which we already know about: Iron Man 3, Captain America 2, and Thor 2. So what are the others?
Feige: "In a week and a half we start filming 'Iron Man 3'…by the end of the summer we'll be working on the next 'Thor' film, early next year the next 'Captain America' film. Those are the three that we've announced so far, but we've got two beyond that we haven't announced yet that we're working on."
