I will also say screw DiCaprio, but only for the role of Black Mask, cause I dont see him in tat roel due to his small physique. BUT I would love to see him in some Batman movie. The man is a great actor. He fooled us all with Titanic, but Aviator, Gangs of New York, and mostly the Departed and Blood Diamond all prooved to us that this man CAN ACT, and is good at it.
The thing is, he has done so many... hmm dont hold me for that word, "respectable movies", I feel he would not be interested in a Batman flick.
As for Black Mask: Man, I would say Daniel Day-Lewis, but I think he would be great in any role, even Rachel

so trying to be a bit more realistic, I would say Sarsgaard or that guy who plays Michael on Prison Break.