Well since the first character is Lobo, I think that the other 3 dlc are going to be:
A) Martian Manhunter (since he is a very popular character in the media, most people know him and it seems to be the perfect choice to complete the cast of the Justice League team from the animated series, also he appears in the stage of the watchtower, same as doomsday in the fortress of solitude)
B) A Teen Titan (We have a roster consisting of three titans, yet another titan would fit perfectly as a ring to the finger! in order to complete a quartet, since there are not too many female characters I think this one is going to be Starfire)
C) Gorilla Grodd (If you guys have finished the game you can appreciate that in the final cutscenes there is an image of Grodd smashing the floor, also this is a game where every hero has its own villain so why not bringing to life one of the flash most famous villains?
D) Black Manta ( To be honest Im not really sure about the fourth dlc I just wish Black manta make it cuz I love him, he is cool! if not maybe Black Canary)
Veterans Day