Spend the indicated number of Justice League of America Shields that you can collect in the game to unlock Bonus Heroes:
Aquaman: 27 Shields
Green Arrow: 76 Shields
Green Lantern (Hal Jordan): 84 Shields
Green Lantern (Kyle Rayner): 55 Shields
Hawkgirl: 70 Shields
Huntress: 53 Shields
Spend the indicated number of Justice League of America Shields that you can get in the game to unlock Alternate Costumes:
Zatanna (classic blue): 25 Shields
Batman (Batman Beyond infiltration suit): 74 Shields
Flash (Walter West): 48 Shields
Green Lantern (retro): 23 Shields
Superman (Earth 2): 65 Shields
Superman (Kandor): 91 Shields
Superman (black recovery suit): 32 Shields
Wonder Woman (biker): 61 Shields
Green Lantern (bold): 30 Shields
Batman (classic blue): 59 Shields
Wonder Woman (toga): 44 Shields
Martian Manhunter (modern): 29 Shields
Martian Manhunter (true): 15 Shields
Flash (Jay Garrick): 21 Shields
Zatanna (Identity Crisis): 38 Shields