I am pretty sure Archimonde is dead.I used to think it was cool, but the fact that no NPC ever actually dies (and stays dead) is starting to get annoying in this universe.
I am sure Van Cleef is dead.
I am sure Malygos is dead.
Pretty sure I can go on.
NPC's like Kel'Thuzzad have a history of coming back... because that's just in their nature.
If these two people are not implemented as bosses in Icecrown, then they will be guardians of the Lich King himself - Saurfang and Morgrainne. Arthas has both of their souls.
P.S. Bolvar is the new Harvey Dent.