Re: Who should be the villain in an Avengers sequel? (Poll)
I'll take the opposite tack. There's really only two reasons why Stark creating Ultron would be a good idea:
1) It's narratively efficient. We know Stark, we know his capabilities, any movie could basically open with Stark doing this. We are already connected to Stark's daddy issues that he would pass down. You get to the story better and quicker, because of the way Stark is set up.
2) It's available. In a dream world Ant-Man will be in the present day, starring Hank Pym, and it will be designed to set up this perennial Avenger for all his Avengers storylines. I'm not convinced that will be the case. At all. And why should I, other than that's how we would do it? In that very likely Scott Lang-driven scenario, there is no real way for Ultron to flow out of Ant-Man. So if we want Ultron, Tony would be the only way, which would also make a Stark!Ultron a good idea, unless you wanted to boot Edgar Wright and hire a new director that will put Pym front and center.