My guess the way this whole thing pans out grows really well out of the ends of both Iron Man 3 and Captain America 2. Tony feels that to protect the world he needs to think bigger than himself as a hero so based on the same AI tech used by Jarvis to run the Iron Man Suits he makes an army of drones with a new AI called Ultron to keep the peace as a replacement for the solders, spys and strike teams lost by the dismantling of S.H.I.E.L.D. The Avengers remain as the face of this organization and conceivably the field generals of these units.
Maybe due to tampering by Strucker or just a cock up by Tony the Ultron Software misinterprets the directive and reaches the conclusion that the best way to police the world is to eliminate the human threat. The Avengers must now deploy to defeat the army of Ultron drones while they try to figure out how to disable the software driving it, Tony has to face the fact that no matter what he needs to suit up, he can't just drive the initiative from the tower completing his arc from Iron Man 3. Which is what makes it a challenge because destroy all the bodies you want until a copy of the software is gone, which could spread out across the net, you still can have more self replicating Ultrons.
Vision gets built and his AI is based on Jarvis.
Other things I expect:
1. Ultron takes over Tony's armours except one he wears into combat. Maybe the way of keeping Ultron out is discovered in the construction of Vision. The Hulkbuster Armour gets hijacked by Ultron and goes after Banner due to his high threat level. The Hulk rips it to shreds. This drives a rift between Tony and Bruce because he did not know about that suit.
Bruce: Why would you build that, I trusted you
Tony: I trust you too, but you're so powerful I wanted something if I needed to stop you
Bruce: You'll notice it didn't work (Bruce leaves)
Tony: (holding piece of Hulk buster) oh I noticed
2. Captain America basically repeats his act from Cap 2 actually saying this is not that different from what happened at Shield and remains the moral compass of the movie.
3. Scarlett Witch and Quicksilver are effectively Inhumans.
4. Post credits is teaser for Dr. Strange or Ant Man
@g_man Folks give Scott a lot flack. All things considered Scott handled the issue with Past Jean well. As far as folks feel he has fallen he still understands the ramifications of his actions. Again Scott leaving Maddie to find out if Jean was really alive was an almost impossible decision to make. Either way he would have been screwed. The psychic affair was bad though.
And as much as Wolverine cares about the students I feel that he is a bit unrealistic. Kids should be kids but in the current environment they are more hated then they have ever been. Learning to use your powers is great but being prepared for the worse is better.
In regards to what @jonny_anonymous mentioned about Kitty being out of line I am on the fence. Kitty has to protect her students but in this instance she heard exactly how Scott responded. She her attitude was just an emphasis that Scott needs to watch himself around Jean just as Jean needs to be watched around Scott. And most importantly Scott & Kitty agree about Jeans training.