I sort of see your point. Nathan would be excellent as the public persona of Bruce Wayne, millionaire playboy, nice guy but not the sharpest tool in the box. But maybe not the bat himself.
Jewel Staite as Harley Quinn? Sean Maher as the Riddler (I'm reaching here)? Mark Sheppard as a younger Alfred? Or, if we're opening it up to other Whedon works, Anthony Head?