In short -
Arthas resurrects Anub'arak, who to redeem himself in the eyes of his king, tunnels deep beneath the newly constructed Coliseum.
After battling our way through whatever the Coliseum holds, Anub will steal Malygos' ways, and will destroy the floor of the Coliseum, leaving us to fight Anub deep beneath Icecrown in Naz'anak - The Forgotten Depths.
I'm unsure if the final boss will be Anub, as the Black Knight, who I believe to be either Bolvar or Saurfang, has a large part to play in this patch.
This is only speculation, working on the information provided at the time of posting.
Discuss.