This is one of the most difficult roles for me to choose. I'd like to pick someone more ethnic to play the role because of the characters roots but I'm not that well versed in foreign actors unless they've already made a big name for themselves.
Right now my pick for the role is Christopher Heyerdahl. Yeah, he's a Canadian white guy but he's a very good, underrated actor that is great at playing villains. He has a great menacing voice and is good with doing different accents. He's played some roles that have a somewhat similar look to Ra's despite being Caucasian. He has a good fierce, evil look to him when the role calls for it. Bit of a chameleon who's played alot of different roles, he's a really good character actor in my book. I think he could be pretty great in the role.