I made a Grave Tusken costume from the video game Dark Forces II - Jedi Knight and am pretty sure I'm the only one who did, so there are still many Star Wars characters from video games, books and such that probably haven't been made yet, plus The Clone Wars is relatively new with many unseen costumes in it too.
Some good looking costume ideas in the Animated Clone series. And as they are newer they will be less likely done to death. Comics maybe another good source for the rarely done. Otherwise I would have to agree with many above, do something you like. It would be a waste of time and money to have it sit on a shelf after its finished.
I would love to see Naga Sadow! He is one of the ancient sith or Tulak Horde. He has many different appearances. Here you can find many of the big empire sith lords of the books: HALL OF THE SITH LORDS
Im sorry, im just really into sith. I really don't know many others.