Unlockable: Various Unlockables

Hard Mode: Beat the game on Normal
Nick Fury: Beat the game on Easy or Normal
Daredevil: Collect the five Daredevil action figures. You should have Daredevil half way through act three. 1. free the guy trapped in a clam (Atlantis). He gives you a code to unlock a close by chest near a fallen submarine, 2. Dr. Strange's library, 3. Beginning of the circus level in Dooms Castle, break down the door to a storage room, 4. Near the Odin statue's left hand in the Bifrost level is a button that will reveal a chest with the action figure, 5. Take the right path in the Asgard level. It's on a small path before you get to the houses.
Black Panther: Collect the five Black Panther action figures
Dr. Strange: Save Dr. Strange in Mandarin's Palace
Ghost Rider: Trade places with Ghost Rider in Hell in order to save his soul
Blade: Beat toy grab minigame in the Carnival in Murderworld.
Second & third costumes for all characters: Must play as that character and gain between 2-4 levels.
Fourth costume for all characters: Beat the Combat Training challenge for the character.
Silver Surfer: Defeat Galactus on the Skrull Throneworld