The confirmed playable characters (so far) are:
Iron Man
Captain America
Hulk
Black Widow
Thor
Miss Marvel
Hawkeye (The first of four free DLC characters)
Spiderman (Exclusive to Playstation, will be available in 2021)
Basically the game will have an expansive story mode and then a bunch of multiplayer options to which the developers want to keep adding free content for the forseeable.
I wasn't sure about the game until recently but now I'm quite excited to play the beta, the beta begins on the 7th for people who pre-ordered the game and an open beta will begin a week later for playstation players with an open beta for other platforms arriving later.