Hey Valentina, firstly cool article. And I totally agree about the depiction of woman and particularly the devolution of female characters in Hollywood and Superhero movies. I just wish you'd chosen some other or more in depth examples.
I mean while I totally get your points from Age of Ultron, I think it's important to say that Black Widow has the third highest screen time in the film and her action scenes are traded for the development of her backstory and the love between her and Bruce. Now don't get me wrong the romance between them is totally chalked on, but at least it was mutual and allowed us to see another side of her.
For me a better and more apt example is how she's portrayed in Iron Man 2. In it, she's shown half naked, is made to play the secretary and is constantly sexualised and over-dressed. Her wig looks like goldilocks dropped in red wine.
And I totally get your comments on Whedon's sexism. But that he doesn't see women as powerful? I would actually say its the opposite. Often his female characters (Buffy, Zoe, River) are powerful and overtly so. It's just they're usually left to play second fiddle to a more 'complex' and 'traumatised' male.
The problem with Age of Ultron is that it doesn't really work as a film. At least not after the halfway mark. Because of this lots of characters start to suffer and she's unfortunately one of them.
Other, and for me, more egregious examples of how badly female characters are treated in Superhero movies actually comes from WonderWoman in basically everything she's in and Captain Marvel.
But anyway, you're totally right about how Margot Robbie is portrayed in SS. I hope the newest film will correct that!