I am level 15 right now, I hope they had something more than just the sim to the game, like some boss battles eventually.
As for heroes from what I'm reading, Black Panther, Female Crimson Dynamo, Squirrel Girl, Hawkeye, Zemo and Tigra are in the game coding right now, so they will be added after the ones already in game.
Level cap is 18 right now, so we can't get all the characters ingame anyway, I think we can only get up to Ms. Marvel.
Seems right now there are no, Spiderman, FF or Mutant characters(the last two won't be ingame at all obviously), so I assume what they will add will be Avengers that don't fit those categories and other characters, I'm hoping for Hercules and Doctor Strange.
I wish the prices were lower, I wanted to buy the premium characters, but its too expensive to get them all, at least to me, hopefully they introduce a way to farm gems, we get like 4 every level up, I only have 94 the least expensive character is Spider Woman at 250.