Now they are scouting out locations, but nothing for the story is final, here are a few ideas, I do not know if Superman will link into Green Lantern, it would be hard, but not impossible, I mean Iron Man was barely mentioned in Incredible Hulk until the very end:
The Return of Zod: Apparently Zod does not die and something happens to him, but he returns and apparently with allies from the Phantom Zone. Likely this one would make a good idea for a third sequel, but not the immediate one.
Mongul: After learning that he is not the only Kryptonian, Superman searches for Krypton (Superman Returns anyone?), and winds up in the closest planet near Krypton. It is the planet Dheron, I do not know if like in Superman: Earth One they blew up Krypton, what is different is that Dheron is the War World that Mongul rules over. So I am guessing that Superman becomes a gladiator on the planet and faces all different kinds of species, would a great tie-in to green lantern, with some GL's being gladiators, but I don't know about that.
Luthor: After Zod, its kind of like you can take on anybody right? That's why Luthor seems like the best choice, Superman does not suspect that a mere man can beat him.
Morgan Edge and the Intergang: This one involves Luthor as well, it basically would revolve around the Intergang going to "war" with the citizens of Metropolis like in The Dark Knight Rises.
Darkseid: Lastly, the most unlikely choice, but what better way for Supes to prove himself than taking down pure evil itself?