The lone female Avenger (until another Scarlet showed up), Scarlett Johansson's Black Widow has been kicking ass and having to put up with all the competing male egos for some time now, and she's really one of the strong points of the MCU (even despite the weird Widow/Hulk love interest arc in Age Of Ultron). She has a compelling and still rather mysterious backstory, and hopefully we're going to learn more about her in the coming films, which may one day include a Black Widow standalone. She brings something different to the team, able to hold her own without superpowers, and is one of only two major female characters at this stage. That being said, she's had strong arcs in both The Winter Soldier and Age Of Ultron (at least in terms of time dedicated to it), and that may be setting her up for a fall. Out of all the characters I'd say could be considered genuine possibilities, hers would be the most poignant death, and Marvel may have that in mind.