My problems with the current Black Widow (coming from someone who isn't a die-hard Marvel fan but is reasonably familiar with the characters) is that she wasn't ruthless enough. When I imagine Black Widow fighting someone I don't imagine ridiculous flip round move, I imagine her either quickly dodging, perhaps dislocating one or more of her enemy's joints and then knocking them out with one precise kick/punch/elbow or, when lethal force is permitted, just stone-cold shooting them dead. I mean it's been pretty good but not quite as efficient as I imagine her being/they seem to chuck in big flashy stuff from time to time just to look "cool". As for her emotional state, she's been through traumatic stuff, but I want her to be so bad-ass she isn't even bothered by it anymore. Like "oh, bad guys tried to strip me of my humanity and I've been trained to murder people without blinking? Yeah, I got over that years ago. What's my mission?", where any vulnerability she shows is just to manipulate those around her. Problem is this sort of character is not very family movie friendly.
In terms of a Hulk 3 movie where she's running away with Hulk like someone else suggested. Yeah, I could see that, but as I imagine her, she would pretend, with such dedication that it'd happen, to be in love with Banner for years and betray SHIELD and run away with him (under Fury's orders the whole time) just so she can drag him back to the team when the time is right even if it means breaking Banner's heart/ruining his life. As in you have no idea that she's putting it on until a post credits scene where you see her in a call with Fury.
In other words, currently she says she feels like a monster. I want her to be that monster. The sort of monster who would kill Captain America and live in exile, on the run from SHIELD, for the rest of her life if Fury asked that of her for the greater good. I want Black Widow to be scary with a bad-ass level akin to that of the Boss from Metal Gear. That character probably isn't the best main character for a movie, but is a damn good secondary one.