While there's no solid release date in the current Marvel timeline, rumors are already spreading about a potential Iron Man 4. Some sources are saying the delay is because lead actor Robert Downey Jr. is asking for a raise, others argue the studio is merely waiting until their current projects are closer to completion before they make any further announcements. Since we can assume that an Iron Man 4 film wouldn't be released until 2019 at the earliest, it begs the question: is making one really a good idea?
The movies are certainly a financial success. Robert Downey Jr. has been one of the highest paid actors in revent years (his salary for Iron Man 3 was $50 million- worth it when you consider the film was the 7th top grossing film of all time), and the Marvel franchise is one of the most successful of all time. But RDJ is fifty years old. In the films, his character has given up on armored heroics, and the impending Civil War movie will likely feature him as an antagonist (if the comics are anthing to go by). So, while the actor is contractually obligated to appear in the next films, that's no guarantee he'll be flying around and causing property damage.
Unlike Spider-Man and Captain America, it doesn't seem possible to cast a different actor in the role. RDJ has defined Tony Starknso completely, it would feel incongruous to cast someone else. Captain America is an identity that has been used by multiple characters, and the Spider-family is always growing (thanks to rogue spiders everywhere), but there's really only one Tony Stark. And as he said: he is Iron Man. They are inseparable. From a storytelling perspective, it seems like it might be time to give the character a rest. While it feels like Marvel is too big to fail right now, we've learned time and time again that NOTHING is too big to fail. Even Tony Stark isn't immune to stagnation. Sure, there will always be fans who want more of their favorite character, but in order to maintain the mass appeal Marvel has earned, they can't continue to target that niche alone.
What they could do is bring back Pepper Potts- as a headlining hero. Her character arcs have been very limited because they rotate around Tony. But give her an antagonist and enough power left over from Extremis, and we'd have a story that is interesting and new that's still connected to the Iron Man solo films.
Another option woukd be to focus on Rhodey aka War Machine. He has the same capabilities as Iron Man, but he's a completely separate character. And, it seems like as of Age of Ultron, James Rhodes will be standing in Tony's place on the Avengers team, so it would feel like a natural progression. The Tony Stark we know and love will still be present- he built the suit after all. But the story would be rejuvenated by the fresh perspective a different character would provide.