Deathstroke

Slade Wilson volunteered for a Captain America-type military experiment, making him faster, stronger, and tougher. Under the name Deathstroke the Terminator, he worked for the military for a while but eventually quit (around the same time his wife shot his eye out). Deathstroke (writers dropped the "the Terminator" from his name when some Schwarzenegger movie came along) went on to a career as an assassin, wearing his executioner hood with the one eye covered, and his highly impractical orange and blue costume. Maybe he's just a Florida Gators fan?

From his military enhancements, Deathstroke is unusually strong, fast, and intelligent, and has a Wolverine-like healing factor that allows him to recover from all sorts of injuries. Except, apparently, for his lost eye. That never made any sense to me. He's also a skilled martial artist, like most every other comic book assassin, but between that and his strength he's able to keep up with Batman in a toe-to-toe fight.

(image stolen from Phil Noto)

Pros: He's an assassin, which works perfectly for where The Dark Knight left off. He does have some supernatural powers, but change his super soldier experiments to advanced steroids and he fits the level of realism in the movie universe.

Cons: For story purposes, he's identical to Deadshot and pretty similar to Lady Shiva, and either of those would probably work better. Deathstroke can also be confused (ie, people think he was ripped off by the creators of) Marvel characters Taskmaster and Deadpool, and even though that doesn't seem like you'd hold it against Deathstroke, it takes away from his uniqueness. He also may be more associated as a villain for the Teen Titans, and if that gets turned into a movie (which I'm sure eventually it will be), they may want to save him for that one.

How I'd do it: Exactly how I'd do any of the other assassins, someone after Batman hires Deathstroke. Batman has to flee from the police, take down the assassin, and make sure no one gets hurt in the crossfire.

Casting Guesses: Behind the mask, Slade Wilson is an older guy with an eyepatch, but still a tough hardened military man. It's a Bruce Willis type role, or Mickey Rourke if they can get him to not look freakish. Although I have no problem imagining Rourke or Willis as Deathstroke, I have a hard time imagining Rourke or Willis fitting into a movie like Batman Begins or The Dark Knight.

Verdict: I highly doubt he'd be a featured villain. But I could also see a cameo Deathstroke appearance, along the lines of the time Darth Vader hired a bunch of bounty hunters to track down Han Solo, except instead of Boba Fett and company, it'd be Deathstroke and Deadshot and friends.