In an interview with the Marvel president Kevin Feige, he sheds light on some burning questions, but no definite confirmations were released.

For 'Iron Man' fans, what is sure is that there won't be any Iron Man sequel directly following Iron Man 3. There were rumors (meaning they were not officially confirmed but everyone believes it) that Robert Downey, Jr. and Marvel had negotiations for another set of Iron Man movies but they did not reach an agreement. That notion was indirectly upheld with Feige's response from an interview with French site Reviewer(translated by Unleash the Fanboy):

"That's the question. And again, that's a question we won't have to face until after Avengers 3. So that's at least five or six years away." 

It is highly unlikely that Marvel would not make Iron Man a part of Phase 3 if they had that option. Iron Man is still their most profitable solo franchise by far. Downey's presence also played a big part of The Avengers' success.

The indication that an 'Iron Man 4' movie would happen in 'five or six years' has possible implications. It is definite that they would not abandon the franchise, but the question is about Robert Downey, Jr. He's not getting any younger, and it's a mystery as to what would prompt him to change his mind in that time span. Will Marvel offer more money? Or would they take a different direction? Is Iron Man just a suit of armor that someone else could wear or is it Tony Stark? Well, Marvel has five years to answer those questions.

In the meantime, there are other solo films that they can do. The Hulk has had two film incarnations that were mostly disappointing. However, their trial run with Mark Ruffalo in The Avengers was met with positive reaction. Marvel played it safe by not getting a more popular, established actor (Edward Norton did not accept the offer). Mark Ruffalo is competent, but he hasn't actually carried a movie on his own.

We might be getting ahead of ourselves, but this is what Feige had to say regarding a Hulk solo movie:

"We have been discussing after Avengers what that would be, what that would look like. It's really Mark Ruffalo portrayal of Bruce Banner, in the way Joss Whedon brought him to life in Avengers that worked. It's even why you ask me that question. Which is good, we wanted that to happen, we wanted people to come out of Avengers saying The Hulk was their favorite character or at least one of their favorites. The fact that it did happen, was awesome. So, you know, all of the effort related to Hulk right know is going to Age of Ultron. It's exciting, because he hasn't been in any other film, so you can expect to have a lot of Bruce Banner and Hulk in Age of Ultron. Where we go after that ? We'll see. It will be a little while before we figure that out but, we do think there's plenty of opportunities with the Ruffalo Banner."