After watching Iron Man 3 I believed that the Iron Man series had reached the conclusion of its story. With Tony having blown up all his suits and getting the piece of shrapnel removed I, at first, went along with many fans in believing that there was no possible way to continue the Iron Man story.
However, some crucial news that has been posted on this site in recent months has led me to change my mind. With Whedon having announced that the title for the Avengers sequel is Avengers: Age of Ultron, and that Hank Pym aka Ant-man is not the father of Ultron; it has been highly speculated that Tony Stark would be the man who builds the Avengers' archenemy. If that turns out to be the case then I think there is room for one more Tony Stark story.
Throughout the Iron Man trilogy Tony's life has been a roller coaster consisting of highs and lows. In Iron Man, Tony experienced his first low point when he was captured by terrorists to build them weapons. However, he came back from that thanks to him building the suit, and giving birth to Iron Man. In Iron Man 2, he hit a deeper low when his arc reactor began to kill him thus leading to alcoholism, and his house being partially destroyed in a battle with Rhodes, but he eventually fixes all that thanks to his father. Then finally Iron Man 3 has him hitting an all-time low with the battle in New York causing Tony to have PTSD, and his house being completely destroyed by the Mandarin, and yet he still manages to crawl out from the pit of despair.
Tony, however, has not hit rock bottom yet in his life. If Tony builds Ultron, and Ultron kills one of the Avengers, as well as a majority of the human population, then this could lead to Tony absolutely hitting rock bottom for Iron Man 4. If Hank Pym is any indication then Tony will be not only experiencing PTSD but an extreme burden of guilt on his shoulders for all that Ultron has done. This could lead us to seeing this kind of Tony in Iron Man 4
This could lead to Tony losing absolutely everything he holds dear including Pepper if his guilt leads him to be an abuser. His extreme guilt, and alcoholism, could even cause him to lose a battle against even the most minor of villains like Madam Masque or Ghost thus leading him to even give up Iron Man for awhile. However like a phoenix rising from the ashes Tony could come back in a blaze of glory to fight off some major disaster. Something I was thinking could happen is that Ultron could return somehow and take over all of Iron Man's suits that he may have built between Iron Man 3 and Avengers 2. This could lead to something taking place in Avengers: Earth's Mightiest Heroes where Tony has to go all the way back to his first suit to fight and defeat Ultron once and for all.
Anyway these are just my thoughts. Feel free to debate with me in the comments section below.