Warning: This article contains spoilers for the Wrathgate in-game event.
So Callouse of Vashj posted a theory on the General Forums that is so elegant that I'm sort of ashamed I didn't think of it myself. At the Wrathgate, we know that Arthas absorbed the soul of Saurfang the Younger into his sword. Evidence also points to the idea that he stole Bolvar Fordragon's body. This has lead to rampant speculation that we will eventually fight one or both as thralls of the Lich King.
But Callouse posits that we may end up seeing one and both at once. That is, we'll see Saurfang the Younger's soul in Bolvar's body.