Tony creating Ultron fits perfectly with his character arc and makes a lot of sense for putting him back into the suit. It would also explain how Joss plans to make the Avengers butt heads with each other: A lot of strife will be born from their de-facto leader being the one to build Ultron. How do they know they can trust each other if one of their own builds a death machine? On top of all that, it creates and greater drama for the MCU itself. Tony being the one to build Ultron would hit harder with the audience. People would care about the personal torment Tony is going through after building Ultron, they wouldn't so much with Pym especially not before he even gets his own movie. It would sort of be like, "oh this random new character created a death machine and I should care about his own feelings why?" I know a lot of us comic fans aren't going to like it but I'm just saying some of us should start coming to terms with it, just in case. Look at the positives of it and how it makes more sense in the MCU to have Tony be the one to build Ultron than Pym. Also remember it's no guarantee Pym will be as big a figure in the MCU as some of think. Remember Wright's original Ant-Man script. It may not be as different now as we thought it was.