Marvel's Avengers is coming out in early September, and some lucky players have had access to the game via the pre-order beta! The beta files for the game have apparently revealed the potential of some well-known characters to join the cast of already available heroes. We don't have a whole lot of detail on them currently, but the fact that they are named in the files likely means they will make their way into player's hands eventually.
According to a data mine of the recent beta for Marvel's Avengers, it looks like some additional characters will likely be joining the cast in the near future. There's Captain Marvel, She Hulk, Kate Bishop, War Machine, and Peter Parker mentioned in the files. We already were aware of Spider-Man being added to the game, which was controversial because it appears that it will be a PlayStation exclusive, but we weren't aware of the other four characters!
Captain Marvel has become a well-known character in the Marvel universe with her popular movie that starred Brie Larson in the role. She Hulk, the female version of the Hulk, apparently received an emergency blood transfusion from Bruce Banner, which then gave her some of the powers of the unstoppable Hulk! Kate Bishop is the female version of Hawkeye, and is part of the Young Avengers. War Machine is pretty much an alternative version of Iron Man, who eventually takes over the role of Iron Man in the comics.
There's a lot of lore here to digest, so be sure to head to Wikipedia if you've never heard of some of these new characters!