Ahhh, gotcha. Hmmm, that is a good point. I don't know, I think there is definitely a fine line, but if Ultron were to be the main villain...I don't know. Thinking film structure wise and in terms of well put together fight scenes, I just think that Ultron wouldn't work as the only villain against an entire team of the world's greatest super folk. It's the same reason that a Justice League movie with only Darkseid as the villain wouldn't quite work when it came to fight scenes. As an enemy for one hero or the leader of an evil force of some sort, it works. But here you'd have one singular entity around the size of a human being attack by 6 or 7 other humanoids. It's hard to create an interesting fight dynamic on screen with that set up. It even bends logic to a degree. If you have half the team with projectile based attacks (Black Widow, Hawkeye, Iron Man, sometimes Cap when he throws his shield), Thor in close with his hammer, and 2 guys who could just pound him into scrap (Giant man and Hulk), and one who could get in him and screw up his wiring (wasp), it doesn't seem like he provide not only enough fights, but none that would be especially "epic" outside of effects. In a movie like this, audiences are going to come in banking that a movie featuring a collection of heroes like this is going to be packing the fisticuffs like no other.
Ultron as a mastermind with some sort of force behind him would work. As I recall, Pym was working on the Ultron robots for SHIELD as expendable soldiers. Maybe when we meet him in the Avengers film, he is already employed by SHIELD and working on them as an alternative if the super soldier project falls through. It works and Cap is revived and they start working on a new formula to give to more soldiers. The Ultrons are effectively no longer needed, but they decide that they will serve a purpose regardless of what their master wishes...
Maybe? I don't know, I think it would work, but past the premise, it starts to sound like Marvel's version of "I Robot", replacing Will Smith's character with the Avengers. I think either way you go (aliens or Ultron), you walk a fine line. With the talent onboard so far, whether in front of or behind the camera, I have faith in the direction they'll take. You're right though. The concept of technology gone wrong does serve for an easier transition for an audience.
The other choice could be to jump staright to an Ultimates 2 type of scenario, with all the hero's respective major villains conspiring to attack the city (minus the black widow traitor aspect. I'd like her to stay hero)...
PS
Sorry for the long posts. I have alot of ideas.

t: