That "sidekick" Samuel L. Jackson revealed he'd be getting in The Avengers? According to Deadline, it's one-time SHIELD director Maria Hill--just like those of you who said "MARIA HILL" foresaw! Furthermore, it seems How I Met Your Mother's Cobie Smulders is a shoo-in for the part, and could well be signed by the weekend.
Outside of her sitcom work, Smulders was also rumored to be Avengers director Joss Whedon's first choice to play Wonder Woman--you know, back when Joss Whedon was making a Wonder Woman movie instead of an Avengers movie--so the choice isn't altogether surprising: if he can't cast her as a superhero, next best thing is casting her as a character in a superhero movie, right? And next best thing after that would probably be finding out her hobbies, then getting one of those expensive cake places to make a nice cake based on them and surprise her with it. People always seem to like that.