Nolan and Snyder have talked a lot about making this a more "grounded" world, so having Supes go up against killer robots, giant aliens, or monstrous mutant versions of himself would probably defeat that a bit.
At least for the first movie. I'm sure we'll see some version of those other characters in the sequel.
Which is why the grounded approach sucks ass. As good as Nolan's Batflicks are they were equally disappointing in some regards because of the limitations set by themselves(!).
I hope you're right and they cut loose for sequels.
I have to admit Shazam! I always crack up looking at your avatar, especially because I have to agree with you in regards to not liking it grounded. I enjoyed the Nolan bat films, but I'm really hoping once this Superman film does well, we can see another baddy besides Luthor in Man of Steel 2.
Hell, I'd have it so that he's in the film but Superman's facing off with Metallo. Introduce Kryptonite, begin the Superman-Luthor rivalry, AND have some excellent fight scenes.