Yeah, honestly, I don't what they will be doing with Avengers 2. Sure, some characters like The Falcon will be introduced beforehand, but if you include Ultron, Quicksliver, Scarlet Witch, pre-Ms. Marvel Carol Danvers, and pre-Ant-Man Hank Pym, that's a half dozen new characters in one movie, in addition to the half dozen main characters we already know. This movie will be insane, and Avengers 3 will probably be even crazier.
Oh, and I think Ultron will either be a creation of SHIELD gone wrong, or a creation of Thanos sent to Earth as a "second wave" attack.