Definately Arthas of course. But what else are possibilities, i know its early but im bored >.<
My thoughts on list besides Arthas
Sindragosa, his frost wyrm from the cinematic.
Undead King Ymiron
Undead Bolvar Fordragon
Possibly a Prince type boss like the week long karazhan event or from the questline in borean tundra involving Thassarian and his sister.
Mal'ganis (revealed in Icecrown that he is alive)
What are your thoughts?