One thing puzzles me, after building up to the Mandarin for 3 movies and adding a twist, why kill him at the end? It does not make sense. That character can go much further, and with a great actor like Guy Pearce then they really can take him anywhere. With his powers and the potential to somehow introduce the ten rings as an actual weapon, he could be a match for the avengers. And I believe he may still be alive. If he can survive a building falling on him and an armor exploding around him then i'm sure a repulsor blast into some rubble will not finish him. Further more, he seemed to be the most powerful most likely due to being one of the first to be injected with it. So I believe he could have survived and may be a villain in a future marvel movie. Which brings me to the other character who I would like to see return and whom I think it is likely they will.
If Maya Hansen created the extremis formula why did she not use it on herself? Sure she had no motif as there was no visible disability and there was no mention and sure she could have thought it was too unstable to use but we never actually see her die. She falls out of shot as we think she dies but who is to say she didn't manage to give herself extremis in her dieing moments. In my ideal world this would scar her face leading her to become Madame Masque. She could be after Tony for leaving her to die. Maybe returning in future movies.
As I final comment (I know he isn't a villain) I would also like to say that I believe the President will return again. Having a constant figure like this in the MCU is a nice way to build continuity and killing him off eventually could have real impact. Plus he can play a huge part in possible events to come such as civil war.
I know it was a brief article but I wanted to share some of my thoughts. Thank you for reading.