Or, why even forge a body for itself to manifest physically, when he/it can *borrow* one...?
I've always been a firm believer and advocate for Extremis Armor showing up in AOU. It's a logical next step for Tony --- IM3 set up Extremis, and Tony has been working towards perfecting his armor to the point that it literally becomes an extension of his own body and mind. That's what Extremis/Bleeding Edge is in the comics: the armor is literally sheathed *inside* Tony's body and under his skin through nanotech, and is controlled by Tony's mind. So, if a powerful AI like Ultron hacked into that, he would literally own Tony Stark/Iron Man. Make a few "alterations" and "upgrades" here and there, as Ultron is wont to do, and voila --- Iron Man literally becomes Ultron. Which is what I believe the point of that SDCC teaser trailer was all along.
(I said "literally" too many times in this post. Literally.)