I'm glad I'm not the only one making this point.
See here for an interview with Joss Whedon and the Avengers cast. Scarlett Johansson's comments are very salient (and yes, again, one of the good things about The Avengers was the portrayal of The Black Widow as a strong, sexy, ass-kicking superheroine who was not overly sexualised - no fighting semi-nude or in high-heels, or with huge breasts leading the way).
I'd like to see Janet van Dyne, The Wasp, appear in the next movie, just to keep it true to the original comics, though it would be interesting to see how they could make her suitably formidable. With her ability to shrink to the size of an insect while retaining the full strength of a human being, plus flying around and dodging things, plus her electrical "sting", she's actually a fierce and dangerous combatant, but her original character was as a ditzy young woman.
When we first met Janet she was obsessed with fashion and handsome men. When the Avengers first battled Kang the Conqueror, her main contribution that I remember was to wonder whether he was cute behind his mask ... and then there's her affairs with Iron Man and Hawkeye and everyone else, and we won't even mention the infamous one night stand she had with Magneto on Battleworld. ... Mind you, I have nothing against fashion or against someone of either sex being interested in romance and sexual pleasure. The main thing is that Janet van Dyne at her worst has seemed like a total airhead who'd be better off doing reality TV with the Kardashians than superheroing.
Still, more recent depictions of the character have toned this down, and we've seen over the years that as The Wasp she can be versatile, brave, and feisty.
Better still, perhaps, let's have Carol Danvers, Ms Marvel. Or call her Captain Marvel, since Marvel owns that name and is apparently going to give her that name in the comics. If Marvel Comics could make Carol Danvers the Captain Marvel in the public imagination, overtaking earlier characters with that name, it would be a huge coup for them. I've gotta admit that I've always found Carol a slightly boring character, possibly because her powers (flying, energy-blasting brick) seem so generic. (Yes, there's more to them than that, but that's basically how they are portrayed.)
But she has an interesting back story and a stylish look. Give her a less blatantly sexualised appearance - surely this is holding her back - while retaining something of the same design, and she could still be a great character. Somehow Marvel has just never quite got there with her, despite a lot of unsuccessful attempts to force her down our throats as one of its main stars. If she were well portrayed in an Avengers movie, she could finally be the breakout character that I assume Marvel has always wanted to her be. It would also be good to see a female character on the movie-Avengers team who can rival Thor and Iron Man in raw power.
Who else would we like to see in the movie Avengers when The Avengers 2 comes around?