I think it be cool to start a character with Sith background instead of Ex-Jedi. Just offer a different player view like the Opposite Force for Half Life. Better yet, he/she battles Raven or Exile in some critical battle at the beginning of game and get knock unconscious. Wake up with every one he/she knew dead and being hunted by the Republic. Start a story of revenge or rediscover history of the fallen. Frankly, KoTOR never tell story of the Sith Movement besides simply they are all evil.