For starters, players are going to have access to 24 superheroes out of the gate. This will help get people acclimated to the Fusion ability -- when two heroes are allowed to combine their powers for a special attack on the enemy (or a special buff for a friend). Any two heroes can perform a Fusion, and each Fusion combination is different; for example, Human Torch and Thor can create a fiery tornado.
Then again, Thor can instead perform a Fusion with Captain America where he bounces his lightning bolts off of Cap's shield. If you're keeping count, that means there could be over 250 possible fusion attacks that you (or your friends, as the game still supports four-player co-op both locally and online) can perform during your play through of the game.