Bolvar Fordragon will take the place of the Lich King following the death of Arthas as his predecessor. Below, I have detailed facts supporting this conclusion.
I am drawing these ideas based off of current content, assuming Blizzard does not randomly change lore.
Currently Bolvar holds no "position". He is able to be moved freely, yet maintains a "good" role. All other suitors have important in game roles and would not be able to be moved without much difficulty:
Tirion Fordring- leader of the Argent Crusade, essential in the DK starting quests.
Darion Mograine- similar to Tirion he is the leader of the Ebon Blade and currently holds a necessary lore position for the DK quests.
Jaina Proudmore- leader of the Alliance on Kalimdor, may be the only thread keeping the Alliance and Horde from all-out war.
Sylvanas Windrunner- leader of the Forsaken, absolutely despises what she was transformed into.