Michael C. Hall, the star of the hit Showtime series Dexter, is a match for a handful of DC Comics heroes and (mostly) villains in the DC Cinematic Universe. Here are some roles I thought he could play. But feel free to leave your own suggestions in the comments.
He can be very mysterious when he wants to be. He can definitely pull off an insane, questionable Batman villain.
The Flash (Wally West)
A break from the obvious villains he could play for DC, Wally West and Michael C. Hall alike are funny, charismatic guys with a serious side when needed. They also both has red hair...
This isn't a very good picture, obviously, but as you can see by the picture below and by the entire Dexter show, he can play both an insane serial killer mad man, and also a nice funny forensic scientist (*hint, hint*: The Flash!).
If you've seen Batman Begins, which you should have, Scarecrow is very creepy and evil. Michael C. Hall is a good choice for that type of role.
A goodish guy, Maxwell Lord is responsible for organizing the Justice League International. Michael C. Hall looks the part, but can he play the part? Maybe.