The Avengers includes a man in an invincible suit of armor, a lady assassin, an archer, a super-soldier, the Viking God of Thunder and a Hulk. Something they don't have, though, is a lot of diversity. That's not necessarily a flaw, but it does kind of stick out that - in a movie that trades heavily on themes of teamwork and mutual acceptance - when The Avengers finally assemble in New York for the big showdown, their onscreen ranks count only one woman, and the only "person of color" is green.
Not that the film is a complete whitewash/sausage-fest: Colbie Smulders' Agent Hill gets a fairly visible role, though she never exits the S.H.I.E.L.D. Hellicarrier and isn't technically an Avenger herself. Likewise, Gwyneth Paltrow continues to play a key support role to Robert Downey Jr.'s Iron Man, but otherwise she's strictly a civilian. And one would be remiss not to mention that Samuel L. Jackson's African-American revamp of Nick Fury looms large over the entire Marvel Movieverse - both because it's true and because mentioning it right at the beginning here gives me the chance to see how many people don't bother to read the article before hurrying down to comments to bang out some variation on "Shut up! They made Nick Fury black, isn't that enough?"
Just as this observation isn't innately critical, it's also not indicative of anything sinister on the filmmakers' part. Superhero movies, by and large, have a diversity problem because the comics they're based on historically had diversity problems. However, movies aren't required to follow every detail of their source material; just because it took a while for The Avengers comic books to (for lack of a better word) integrate doesn't mean the movies have to wait, too.
With that in mind, here's a quick selection of characters from the House of Ideas' vault that might be useful in helping future Avengers installments become - if nothing else - a little more reflective of the ever-broadening audiences that have embraced them to such an unprecedented degree.
Overheard at a recent Avengers showing: "How come they didn't ask Black Iron Man to join?" Guy kind of has a point.
Jim Rhodes (aka "War Machine") is a staple of both Iron Man and Avengers comic continuity, and played a major supporting role in both Iron Man movies. When you think about it, given the recurring story point that S.H.I.E.L.D.'s problem with Iron Man is strictly Tony Stark's bad attitude, the idea that they didn't recruit the guy who has all of Iron Man's powers, more onboard weapons and is already an enlisted military professional is something of a plot hole.
Aside from being a cool character that audiences are already familiar with, War Machine offers the same technical benefits that Iron Man and Hulk do. Because their "action scene" forms can be accomplished via CGI and stunt-performers under the mask, it's that much easier to schedule the actors.
Obvious choice is obvious. Gifted with bulletproof skin and unbreakable everything else by an experiment conducted on him while in jail for a crime he didn't commit, Cage (aka "Power Man") was a 70s Marvel fixture who re-emerged in the early-2000s as a prominent figure and high-ranking member of multiple re-tooled Avengers teams. People have been asking for a Luke Cage movie since Marvel started this ball rolling, but for a while it looked like he'd instead be making his debut in the planned Alias: Jessica Jones TV series. But since that show is no longer moving forward, (he appears to be free again - maybe for his own movie, maybe for Avenger-hood.
The biggest plus with Cage as a feature film character, from where I sit, is that he brings a ready-made story arc with him. His original shtick in the comics was being a self-titled "Hero For Hire" - rather than wander New York looking for wrongs to right, Luke Cage worked on commission. Guy who's in it for the money finds something more valuable to fight for? There's your movie(s), right there.