I really dislike the idea of The Rock playing Beast Rabban. The reason I like the idea of Vin Diesel in this role is partially because of his voice. The deep, gutteral sound of his voice makes him sound very animal-like. It also gives the impression, to me at least, that he's not very bright, which fits with Rabban because Rabban ISN'T very bright and as a result the Baron keeps him on a short leash, so the air of contained fury works to Diesel's advantage because all his character wants to do is run around and slaughter Fremen. A few other names for the role of Shaddam who might be good are Christopher Plummer, Hugo Weaving and Christopher Lee.