Arthas needs a victory.
How shall he get it?
Kill off someone important? This works, but, I can't think of someone important enough but who I also don't want to die. Perhaps that actually means there are some perfect candidates such as Thrall, Jaina, or any of the Bronzebeards. Killing Varian or Garrosh would be a victory for everyone but Arthas.
Destroy something? Sure. But what? I think Orgrimmar is a good candidate. Send in the abominations and frostwyrms and a few liches to wreck havoc. Burn down half the place. Fill it with corpses, rising up again. Basically turn Orgrimmar into Stratholme. Force us into some sort of exile city nearby, perhaps the Ashenvale logging camp, it has a decent fortress. We can retake Orgrimmar at the end of WotLK after Arthas has fallen, but even then, it won't be easy.
Another good target: Dalaran. Blast off a huge piece of it and make sure plenty die from the fall. Make them not stay properly dead. Combine that with an invasion from Icecrown to extend a blighted area into Crystalsong. Make the mage bubble under Dalaran a three way battle between the magi, Scourge, and blue dragonflight. Or perhaps by then the story will have advanced far enough that Malygos is defeated and the blues are without a plan, so they are again fighting for survival. The rest of the city will be more crowded and disorganized. For the perfect touch, some of the Scourge on the ground should have the names of missing vendors.
Admittedly these are both mere setbacks, but they are at least something. When was the last time Arthas really messed up anything? Let's see... Oh, he captured Drak'tharon Keep, and then we promptly killed his servant. Then he um... broke the floor of the coliseum and Tirion had to pay a ton of money to get it rebuilt.