I'm basing this all on my own headcanon (so YMMV), so I have Natasha as a survivor above all things, and she's ruthless in that, which could be a Slytherin trait. I guess she could be ambitious in that she's learned she needs to be the best in order to survive. And I do see her taking a certain level of pride in her skills.

I guess I see the ability to be cold, calculating, quick thinking and to a certain extent emotionally detached from the task at hand (not that Natasha is inherently emotionally detached, you know how much I hate that trope!) as Ravenclaw traits, which is why I go between Slytherin and Ravenclaw for movie!Natasha.