Well, unless Kreia was lying, the True Sith are waiting in the unknown regions. I'd be quite dissapointed if that didn't get addressed in K3. I think it is unlikely that K3 will be about the Vong, unless everything Kreia said about the "true sith" was a lie. I mean, Canderous didn't know what the Vong scout was, and he clearly mentioned that "The Sith" had put pressure on the Mandos to attack the Republic. Since Revan's Sith Empire didn't exist yet, this line is most likely referring to the true sith. Logically, since Canderous knew about the "sith," and the (presumably) Yuuzhan Vong scout completely took him by surprise, we might be able to eliminate the Vong as a candidate for being the True Sith.
I'd kinda enjoy having the Main Character in K3 being a "True Sith" apprentice of some sort, who chooses to break away from the Empire and help Revan fight against it. Just an idea.
Wer mit Ungeheuern kämpft, mag zusehn, dass er nicht dabei zum Ungeheuer wird. Und wenn du lange in einen Abgrund blickst, blickt der Abgrund auch in dich hinein. -Friedrich Nietzsche, Jenseits von Gut und Böse