FuryofFrog
the Neon Night Rider
IMOLobo is one to appease hardcore fans and that's it (as far as DLC apparently) with three casuals (including Zod as it's a MoS tie-in thing). If they're trying to maximize profits for a potential second round of DLC then they should have the most well-known or recognizable people that casuals will buy: Lobo/Zod are wild horses; Batgirl, again, is not gonna bring in a significant amount more than Martian Manhunter; Scorpion will sell.
"Cult following" does not equal great sells.
Lobo - Boon probably loves him. He is ultra violent, hes a riff on the extremeness of Marvel, he has many little attributes that Boon personally loves. I'm sure a guy like Scorpion could have been inspired by the likes of Lobo.
Batgirl - Not an obvious choice but she is one of the best characters in the DC pantheon currently. She is not simply eye candy, and watching her recover from her PTSD has been a really cool journey so far. She is a character that women can read and not feel over powered with cheese cake and stuff of the like. The fact that shes is in Injustice reads like an apology from DC for trying to throw Gail Simone off the book, a love letter to the female audience, a way to cash in on Bat Family, and a personal developer choice (Someone in NRS loves 60's Batgirl)
Scorpion - Not only is he easy to do, he is the mascot for NRS. Its not for MK guys its for the NRS guys. Got that Scorpion stamp of approval. Its putting that Netherrealm "We made this, you better recognize" and also probably a test to see how an MK character works with Injustice mechanics such as back to block. Possible future for MK, who knows.
Zod - He is in the new Zack Snyder Superman movie. He is Zod. You will kneel before him.
But seriously, Superman is a hero that WB has been trying to get on Batman level for years. The fact that Batman is still their best character to market is a personal fail for DC and so more aggressive Superman is needed to boost popularity. Nostalgia likes the Terrence Stamp Zod and new Zack Snyder Zod will bring in people who will want to play as him when the movie comes out. What other Superman villains would you like to play as? Darkseid? Bizarro? I think Supes rogue gallery is pretty weak and so this makes for a strong choice.
I would hope that my logic in this is accurate. I also hope that after these characters a round 2 will show up based on fans alone. Wishful thinking but I can hardly damn NRS especially after Injustice is so enjoyable in the first place.