Honestly, I'd love to see Lex in the next one, and I think it's fairly inevitable that, in some capacity, Lex will play a role. Perhaps not the primary adversary, but he's going to be there. I hope we'll see him sport some henchmen, but I also hope that Lex presents a psychological and intellectual challenge to go along with the action. After all, Zod was a bad-ass, but his evil plan basically consisted of blowing up ALL TEH THINGZ!!!
Now, Metallo is a pretty safe bet, since he's easy to translate to the big screen, and seems pretty believable. And, unlike what a lot of you guys are saying, Bizarro is also a likely candidate. He'll have similar powers to Superman, and film makers love making characters into "dark shadow" archetypes in comic adaptions. If anything, Parasite seems the oddball choice, since he's a stranger character, his powers are more unusual, and, like Bizarro, he'll end up having similar powers to Clark.
I think Supergirl is a must, if not only to add a lady to the action again. Faora was kick-ass in Man of Steel. Even my gf, who liked bits of the movie on the whole, loved every scene that bad-ass was in. Point is--we need a bad-ass girl, and we've already set up for her.
One thing I think would be really cool, however, is if Lex Luthor mentions, on occassion, needing to go into a meeting with that idiot Wayne...