Probably a skin for the actual ps3/xbox game

There are actually a number of skin unlock for the console version from completing different tasks in the iOS version. So far the major skins:

Arkham Harley skin - Unlock the Joker in iOS (48,000 points)
Batman Beyond Skin - Unlock Batman in iOS (182,000 points)
Bane Knightfall skin - Rate the iOS game
New 52 Green Lantern skin - Complete a Bonus Battle in Injustice iOS
Hidden reward - Complete all battles in Injustice iOS

There is a chance to unlock both batman and the Joker in a booster pack for cheaper points but the rewards are random in the booster packs.

I'm thinking the hidden reward is possibly Superman Godfall skin

Well I shall start playing the game again, I kinda gave up as like most iOS games is pretty basic