Willow Rosenberg (Buffy The Vampire Slayer)

Prior to the inevitable arrival of Doctor Strange, the Avengers don’t really have anything in the way of pure magic in their ranks – the closest things are Thor and Scarlet Witch – and Willow Rosenberg would add some much-needed power on that front.

Willow is an extremely powerful witch who has teetered on the side of evil – just like some of the Avengers have, so she’d have something in common with them – but, when she’s good, she’s an extremely valuable asset for any team of heroes.

She also has the ability to imbue other with power, which could be important, and her intelligence on a variety of subjects could come in extremely handy when it comes to aiding the likes of Tony Stark in his work in the fields of science and technology.