How's this:
When you start a new game, you create three characters: new PC, Exile, and Revan. You select gender, dark side/light side, character class, portrait, etc.
Spend the first part of the game as new PC, searching for Exile. When you finally meet her, she joins you if you are of the same alignment. Otherwise, you must fight her, defeat her, and convert her to your cause, and then she joins anyway.
New PC and Exile split up to look for Revan. The party splits up between them. You now play as the Exile, until you find Revan. The moment you find her, you switch to playing Revan instead: the Exile must now join you willingly (being of the same alignment) or you must fight her, defeat her, and force her to do so.
You play as Revan searching for the Final Threat (whatever it may be), while presumably the Exile and the New PC are off on the same mission, on different paths (cutscenes showing their progress would be nice). When you finally get to the ending, all three come together for a final confrontation that requires the ridiculous amount of power that the three of them have by this point.
Well?