I think Arthas send off was good. What else should happen? A ghost of him appear and go like "lol forgive me"? Or more Garrosh style "I regret nothing"? Get him send to Arbiter again for some happy lil redemption? A battered, amorphous soul remnant dissipating to a moody melody was great and did not ruin his ending from the past. Now, if it only was not Sylvi who did the send off...