I want Ultron to be saved for the Avengers. To me he makes a perfect Avengers bad guy. Hank Pym is mentally ill. He resents his team, his wife, himself, and everyone. He doubts himself, and his ability to be an Avenger. He also doubts if he is good enough for Janet. Whenever he changes his costume identity it is a sign of trouble. He is like the movie "A Beautiful Mind" but with superheroes. During a break down he creates Ultron, and uses his own brain scans as a blueprint. Ultron becomes the living embodiement of all of Pym's resentment, fears, hatreds, sorrows, all of his dark side put into a robot form. Sort of like in Dargon Ball when Kami tried to destroy his evil side and in turn gave him a form.
Anyway, Ultron is Pym's dark side, his Mr. Hyde, his Hulk. Ultron is also like Pym's son, his son who turned bad. Ultron then attacks the Avengers, who thought that Pym was their friend and colleague. Ultron, to destroy the Avengers, creates the Vision. But the Vision choses to side with the Avengers and his grandfather, Pym. Ultron is stopped and Pym is semi redeemed in the end.
I think it works better with the Avengers as Ultron's targets than Pym and Janet alone, or Pym and Janet with random SHIELD agents. I think if Ultron is Pym (to an extent) then having him attack Pym's closest friends and colleagues in the avengers adds to the attack. Adds a level of mistrust. That this guy introduced in the Avengers as a hero and a sort of boring good guy actual had secret resentful feelings towards them the whole time. But in the end he can be forgiven because it was sort of his mental illness and he/and his creations creation (Vision) help save the day. He could still be voted off the team like they did when he hit Janet, perhaps.
It would also work better in the Avengers because Ultron versus Ant Man/Giant Man and Wasp isn't as much of a threat as Ultron versus Pym, Wasp, Captain America, Thor, Iron Man, Hulk, Hawkeye, Blackwidow, all the avengers at one time. It also introduces the idea that these superheroes aren't just bland good guys, but could have serious problems of their own.