I have wondered the same thing. I think it would be cool to play a redeemed sith warrior or something.
An Interesting Thought My Brother From The Light
I guess if you think about it... the question works for all the different classes:
A Redeemed Sith Warrior Turned Knight, or Sith Inquisitor Turned Counselor...
A Corrupted Jedi Knight Turned Warrior, or Jedi Counselor Turned Inquisitor...
A Smuggler or a Solider That Decided To Work For The Empire...
or
A Bounty Hunter or a Imperial Agent That Decided To Work For The Republic...
Very Interesting... Someone should ask Bioware this question, to see what response they give