Better yet, have an alliance of villains that fit into each character's historical rogues gallery, and have a movie with three active protagonists. Bruce would very much be like their Q, but I think he'd be much more involved than just that (and more involved than just being a win button at the end) if say, the Leader is causing trouble.
But if it's equal parts a Hulk movie as it is a Black Widow and Hawkeye movie, then you'd probably have to bring Betty back around. And if Bruce x Natasha really is going to be a thing, then things are going to be awkward. Unless they can cleverly subvert it by having Natasha play it cool (she is a master spy, and that means being able to act like you have no involvement with something or someone), secretly admire the chemistry between Bruce and Betty, and then maybe break things up between her and Bruce for their own good.
But I dunno, the hand touch thing doesn't really confirm anything. Natasha being protective of Bruce/Hulk isn't the same as her being attracted to him. So it's likely just a rumor and nothing more.