9. Clayface
What we definitely don't want with Superman vs Batman, is for Snyder and co to trot out a villain who has already been done to death (more of that later,) so if the plan is to mine Batman's Rogues' Gallery for a villain who could wander into Metropolis and cause a lot of mayhem, without Superman being able to take him down single-handedly, the solution might well be Clayface. His shape-shifting ability would make him able to hide in plain sight, or at least to require the work of the World's Greatest Director (hence calling in an older Batman for support), and it would also allow Snyder to play with any number of villains to spice things up, with Clayface simply pretending to be the required rogue.
Why He'd Suck He's a great character, and has been involved in some pretty engaging storylines, and the idea of him using his shape-shifting to manipulate Superman and Batman (perhaps against each other) is a nice one, but logistically, Clayface would be a major problem. Fully CGI characters are nothing short of a nightmare: DC should already have learned with Green Lantern that audiences find it hard to emotionally invest in something that was drawn up completely on a computer, which wasn't done so by Pixar. And in Parallax, we also probably have a pretty close approximation of what Clayface might look like out of his various disguises - basically a giant walking turd. And though the Arkham games showed that the character can be pulled off (in very small doses at least) a cinema screen would be pretty unforgiving for the globular mass, no matter how sweet his powers might be.