Arthas will be slain by the Burning Legion.

Or at least that seems reasonable...

Similar to the Illidan encounter, a different force will show up and help you kill Arthas and the only ones whom I can imagine to be stronger than Arthas, and have played a crucial part in the lore so far, is the Burning Legion.

Why would the Legion kill Arthas? Well this is certaintly not the first time Kil'jaeden's tried to destroy the Lich King and it makes sense that the Legion, fearful of the Scourge's growing threat, would want to see Arthas dead as well.

Similar to the Overmind from Starcraft, The Lich King has simply grown too powerful, so powerful that he is a threat to all of existence, even the Burning Legion and of course the mortal races, who'd then join forces to destroy him

Of course there are a lot of other possibilities, but it's obvious by now, that Blizzard intends to take the main characters from the old Warcraft games, especially Warcraft III, instead of the old gods, aspects and such, for future bosses, races, classes.

Just look at the draenei, the blood elves and now the Dead Knight.