Yeah, I think that's certainly a possibility. However, there are two reasons I disfavor it. One, the first Avenger ended up being very Stark-centric, so it wouldn't be a bad idea to mix things up. Second, you know Stark is essentially well-meaning. His creation of Ultron doesn't have the same connotations that Pym has. Pym's an asshole. He built Ultron for assholic reasons. It backfired and blew up in his face. I certainly think they could do interesting things with Stark creating Ultron, but it won't have that same quality.