Re: Characters you'd like to see in future Avengers movies?
To make Ultron interesting, a big chunk of the movie would have to be about Hank Pym. I dont see that happening. Not in TA2 anyway.
They can't even cover the story in the Ant-Man movie since the movie will be about his shrinking technology. And even if they did cover Ultron's backstory in Ant-Man, Joss Whedon would never write a TA2 story that absolutely required you to see another movie first in order to follow.
Best case scenario is that they introduce Ultron as a friendly AI in TA2, subtely foreshadow his malevolence toward the end, and then have him be a villain in TA3.
Personally I dont care if they ever get to Ultron as I think the whole "AI turns on its creator" storyline has been done to death recently. Dont get me wrong, its a great modern iteration of the Frankenstein story, but in the last twenty years or so, its been done over and over again in various mediums. As I mentioned before, only time travel has been used more of late (the Terminator movies combine both of course).