Originally Posted by Chewy
The reason Ultron works as a villain is that his existence takes a relatively good man and ultimately destroys every meaningful aspect of his life. Pym is the ultimate tragic hero.
No way would Marvel be willing to deconstruct Stark like that, nor should they. And without that aspect of the story Ultron is just another killer robot.
The problem they face is they already have a character with an advanced AI system. From a story-telling standpoint, to introduce another one in the MCU would be recycling.