I'd be quite happy with the 4 you suggested and no more.
Darkseid's already gonna be in Injustice as a boss, and I'm fine if he's not the exception to unlockables as well. You can count on him being more than a stage transition in the Hall of Justice. He's motherflippin' Darkseid.
Men only think of the past before they die to see if there was proof that they were ever alive. ~ Jet Black. Cowboy Bebop
A pleasant surprise would be 6 DLC characters because that would be in tune with how the roster slots are currently designed. All they would have to do is shrink the slots to add 6 underneath the launch characters. Although I'd be very happy with just MMHunter and Lobo.