Having Mr. Stark being solely responsible for creating Ultron would prove to be an interesting twist, given how movie-verse Tony has mentioned about the Manhattan Project & Oppenheimer in the previous Iron Man films. And with Ultron's first appearance, he acted in a way that is best described as a "hypnotized Edwin Jarvis," so having the movie-verse Ultron corrupting Tony's artificial intelligence would both prove to me a neat nod to the comics.
However, I do hope that it would be explained that it was created by Hank Pym. Or at the very least, having a couple of scenes showing Hank making Ultron, or even someone mentioning that "Hank took things a little too far in building this thing." would be perfect. And given that the next movie is the Ant Man film, it would be a great justification for why Hank suits up, as well as being a transition for him being an otherwise carefree guy into a man that hates himself, as well as developing an arrogant personality.
However, I do hope that it would be explained that it was created by Hank Pym. Or at the very least, having a couple of scenes showing Hank making Ultron, or even someone mentioning that "Hank took things a little too far in building this thing." would be perfect. And given that the next movie is the Ant Man film, it would be a great justification for why Hank suits up, as well as being a transition for him being an otherwise carefree guy into a man that hates himself, as well as developing an arrogant personality.