If you wanted to include Revan and the Exile in K3, it would be nice if you could play as either one you wanted to. You could even choose the back story for them like you can pick Revan's back story in K2.
If you pick Revan, the story picks up directly after K1 and progesses through years to the point where eventually you would meet the Exile who would become an uber ally and you finish the story out.
If you choose the Exile, then the story picks up directly after K2 and you go in search of Revan, after you find him he becomes an uber ally and you finish the story out.
In either case, both characters start at level 20 and can progress up to level 50 and the backstory you chose would affect the starting allignment of your character. Whichever you choose, Hero or Villian, makes you start with a definative LS or DS allignment. Not all the way to mastery, but a little over half way between neutral and mastery.
Or maybe instead of the unplayed character always joining you, maybe you could choose the back story for both Revan and the Exile and when you meet up, if both were of the same allignment you join forces, but if you chose different allignments, then you will most likely have to kill the unchosen character. Only a very high charisma/persuade character would be able to convince the unchosen character to switch allignments and join up.