Yeah, read about that yesterday, or the day before. I don’t know how I feel about it though. Obviously Arthas is going to appear in some form, and if he does turn out to be an endboss in a dungeon then blizzard has balls, but Arthas being a lich, he can resurrect himself, just like Kel Thuzad. So players will probably never be able to kill Arthas, but come very close. I don’t think Blizzard wants to kill off Arthas in World of Warcraft, or perhaps nearing the end of WoW they’ll decide to throw on an event, huge battle with NPCs and both factions ally themselves and all where Arthas is ultimately destroyed. I can see Deathwing returning as the ultimate evil in the world in the next game, they’ve left it open for him to return, for those not that into the lore Deathwing is the father of Nefarian and Onyxia. The rise of the Deathwing, I could see that happening.
In some time, when they’ve built up enough story, and characters (since many major characters have been killed off in WoW), they can return with yet another MMO Warcraft. Knowing Blizzard I think they’ll wait a couple of “normal” games before they do another MMO Warcraft. The story that they built up to in World of Warcraft took 3 games and a couple of books, and for another WoW to be at all interesting I think they need to build up to WoW 2 in the same way.