As lame as people make him out to be, Aquaman may arguably steal the show in Injustice. I hope that shark is part of his Super. Looks too awesome not to be.
I gotta say it's surprising Doomsday will be in this. He beat the Justice League with little effort, practically killed Superman and even gave Darkseid a run for his money and was only stopped by forces powerful enough to incinerate planets and bust galaxies. I imagine he'll be the Sindel of Injustice and kill off a large portion of the roster in the story.