Let's assume tomorrow the main Alliance and Horde leaders would die. Instantly. I don't know how and I don't care, maybe they were all at some peace meeting and they ate spoiled food or something.
So the following: Varian, Jaina, the 3 dwarves of the council (Muradin is still a diamond, so while not dead, he won't be a contestant), Gelbin, Tyrande Whisperwind, Malfurion Stormrage, Genn Greymane, Velen, Aysa Cloudsinger, Thrall, Garrosh, Vol'jin, Sylvannas Windrunner, Baine Bloodhoof, Lor'themar Theron, the goblin leader whose name I forget, the pandaren leader whose name I also forget are all dead.
Who would you want to be the leader of each race from the remaining characters of importance of each race? And which of them should lead their respective factions?
Some ideas for each race of respectable people left:
Humans: Anduin, Danath Trollbane.
Dwarves: Kudran Wildhammer, Brann Bronzebeard
Gnomes: Toshley, Fizzcrank Fullthrottle
Night elves: Shandris Feathermoon, Jarod Shadowsong
Worgen: Tess Greymane, Darius Crowley
Draenei: Farseer Nobundo, High Priestess Ishanah
Orcs: Nazgrel, Gorgonna (Saurfang and Eltrigg are too old relating to orc culture of leadership, they can remain advisors)
Trolls: Rok'han, Master Gadrin
Tauren: Hamuul Runetotem, Sunwalker Dezco
Undead: Master Apothecary Faranell, Galen Trollbane
Blood elves: Halduron Brightwing, Rommath
Goblins: Boss Mida, Gazlowe
For pandaren there are not enough faction alligned important characters left to think of 4, 2 for each faction...