You have to have the original Core Set to play, but then additional content is all optional. There are hero packs that can add another hero deck to play, scenario packs that add a new villain to face, and campaign expansions that include a couple new hero decks and multiple new villain scenarios.
If you don't care for deck-building, the hero decks are completely playable out of the box. And the game does a good job of having different heroes feel unique rather than just some kind of re-skin. If you do like deck-building, hero packs have interchangeable cards and the more you get the more possibilities. The villain scenarios also have some customization in that there are modular sets that can be shuffled in or out to change the threats faced.
There is no exact release date but the X-Men expansion and hero decks are expected to roll out in Fall 2022.
Below in the spoiler tags I will put all available heroes and villains if you are curious
- Spoiler: show