Honestly Ultron seems like a horrible fit. With no Ant Man character you have to drastically change things around and when you start tinkering with comic book fans expectations bad things can happen. Thanos was a great universe/world villain who could have opened the door to villains like the Skrulls which are more interesting story lines (in my opinion) than the Ultron one. Also, basing this movie all around Iron Man seems like a cop out. You already have secondary characters who's movies aren't doing as well, so why not put the spotlight on them?