Over on the Marvel’s Avengers subreddit, user 6plus4equals52 datamined the on-going beta file, and found 15 heroes mentioned in the game files! You can check out the game code via the Pastebin to scour it yourself, but for those who don’t want to, read on for the list below.
Ant Man
Wasp
Vision
Black Panther
Dr Strange
Falcon
Mar-Vell
Mockingbird
Quake
Winter Soldier
Scarlet Witch
Captain Marvel
Kate Bishop
She Hulk
War Machine
Note that according to the dataminer, these are playable heroes and as such, have their own Challenge Cards, which are only available for heroes that can be played, and whose challenge can be completed.