That's exactly the role I see Daniel Bruhl's character playing, and a way to have a "villain" without taking away from the hero vs. hero conflict. I do think it shouldn't be a large team of villains though. It would definitely have Crossbones, but as far as any others, I think it would be cool if they used Abomination. He could fulfill the role of the Thunderbolts and Ragnarok from the comic, being unleashed in the middle of a battle, probably killing someone Goliath style, and leading to a lot of people really questioning Tony and his methods. Plus it would be a cool call-back without having to spend too much time recounting The Incredible Hulk if Marvel wants to try not to reference it too much.