Ok, i gone over the attack on light's hope chappel and got an interesting thought: Tirion will die at some point, Darion wil take the sword and kill/defeat lich king and cleanse himself (sorta), at that point the spirit of Ner'zhul is banished from Arthas' body and you fight Ner'zhul that way, dunno what happens to Arthas... maybe a vessel to Sargeras, Sunwell style or some followup to next expansion?
Just look over the fight again, there is an interesting part where Darion's father tells him that he will wield the ashbringer against the scourge etc...
EDIT: ok my logic is flawed, there is no Arthas or Ner'zhul anymore, there can be only one... king!
[SPOILER]
They have merged, yes, but theres a quest in Icecrown where Tirion says "Fuck it, you're lost to us Arthas" and destroys Arthas' heart. This means that Arthas has a physical form, and that can be killed. Ner'zhul is still a psychic entity and I'm sure it will be a multiphase fight that culminates in Arthas' death, but have an Algalon-esque Hard Mode where you have to kill Ner'zhul before he escapes back to the Nether, or opens a rift to the Burning Legion, or something.