All great assumptions, but it all comes down to good writing. But as it is, this thread just convinced me how many are so quick to believe that Marvel characters are so over the edge with killing when all I can think of are the X-folks and Punisher.
As for a good Batman analogue, it would either be Spiderman, or Daredevil... Spiderman already has his Joker/Luthor under the guise of Norman Osborn... meanwhile skill for skill, Daredevil is a near match to (or he outmatches ) Batman. And Matt Murdock is a bit loopy in the head (like Bruce Wayne, with his never-ending obsession of fighting crime, literally).
But in order for the Joker to work, you need a straight guy to counter his wild personality, which would mean Daredevil is the best character to play against the clown prince of crime.