9. Jane Foster - Thor: The Dark World
In the comics, Jane Foster is a super interesting character. She's a nurse that later becomes the All-Mother of Asgard, one of the members of a council of realms, and eventually Thor herself. But in the film series she... thinks Thor is hot and then goes into a coma.
Natalie Portman has said, after The Dark World, she wasn't going to come back to Marvel because her role was... Well, her role is she finds a magic thing and then it is inside of her, and Thor has to protect it. She is literally a sexy McGuffin.
The Dark World would have worked much better without Natalie Portman appearing as basically a rather large purse for the Reality Stone to chill in. It feels forced and offensive.