I'm not sure that going from Thanos to Ultron would necessarily work in a movie series. Seems like a giant scaling back of threat, like trying to go from Darkseid to Prometheus. Never know though.As far as Ultron and Ant-Man are concerned, imagine this: have Ultron be his server robot or something in the Ant-Man movie. He does something towards the end to protect Ant-Man, but does it in such a horrifying way that Pym shuts him down. Credits roll. We return to Ultron...booting back up. Boom, Avengers 2 (or 3).
You've seen the first movie, right? The bulk of it is him feeling disillusioned with his life because his technology being used to hurt innocent people, the very people it was being used to save. He felt so responsible for these atrocities that he built an experimental suit of power armor and handled them personally.Tony's personality is all wrong for feeling guilty about Ultron... i mean you might as well have him slap Pepper for how much sense it would make...
Let us now imagine that he helped to create an android, one meant to protect the world should The Avengers ever fall or become brainwashed. He imprinted that android with his own brain patterns because when it comes down to it, there's no one else he completely trusts to protect the world. When that android comes online, it's all brilliance with no conscience. All narcissism with no humbling first hand experiences. It embarks on Tony's mission from the first movie but on steroids - disarm and pacify the entire populace of Earth. And because of Tony, it might be able to do it.
I'm betting he'd feel guilty as sin, and it would be completely in character.
There are some things that work great on the comic book page but not the silver screen and that is almost undeniably one of them.He was all wrong for the role because he looked like a big, buff bodybuilder instead of a fat looking guy who's stronger then a sumo wrestler...