4. Gary Oldman
If Hugo Weaving's busy, there's no denying that Gary Oldman would make for a brilliantly terrifying antagonist, perhaps an Emperor character of some kind. But unlike Weaving, it's best to keep him out of make-up, because he has such an expressive, malleable face. He's one of those few versatile actors who can convincingly play villains or heroes, though it has to be said that he always seems to be having more fun playing the baddie. Who can forget his gleefully over-the-top, psychotic performance as Stansfield in Luc Besson's Leon? While he might have to rein the excesses in a little bit so as not to upset the Star Wars fans - or scare the kids watching - if he can harness some of that manic energy into the Sith, then he could become one of the series' most iconic villains, even if he'll have the massive shadow of Darth Vader to try and escape from (and in honesty, like anyone else, simply wouldn't succeed all that much).