This, all the lore behind Tirion shows him a factionless bro. He is cool, but he remains that way by being his own side.
Also, Tyrande Whisperwind.
Her thousands of years of leadership > everyone else lololol.
Though I guess there is Velen but /snore he never talks. =/
To be honest, I wouldn't chose Velen. I mean his first thought when the Cataclysm started was "yea, how about we flee?". I can understand his reasoning, but when fighting the Legion you need to take a stand at some point, and he was not willing to do so.
As I said, council would be best. One representative from each race:
Jaina Proudmoore from the humans (she is the one that united many of the Alliance races originally and she has shown great wisdom from a young age).
Genn Greymane from the worgen (or Darius Crowley if Genn decides he's too old for this, Genn fits well the role of Saurfang for the Alliance, he had a past that came and bit him in the ass with him sealing himself from the world just as Saurfang drank demon blood and their sons died or Darius's daughter since she has shown she is willing to fight for her lands and her people).
Shandris Feathermoon from the night elves (while Tyrande led her people for 10k years, she is too quick to judge sometimes and her businesses with Elune keep her occupied, and Malfurion is neutral, Shandris judges the situation and has a network of spies everywhere, she's the Mathias Shaw of night elves only more badass).
Moira (the other Dwarven council leaders wouldn't dare leave from Ironforge and let Moira deal with only one other, but they do know that outside, when she's not fighting for power within the dwarven council, she'd fight for the will of the dwarves, which make her an ideal member of an Alliance council, as it's a win-win, Moira fights for the dwarves with the decisions she wants, but she doesn't fight the other dwarf clans for them).
Gelbin (there is no other, unless they develop one, gnome leadership and lore suffers).
Nobundo (Velen is too neutral and preocupied with his visions, Nobundo is the hard stance the draenei need).
Essentially these figures would become the leaders of the Alliance in a way.
Jaina would be the leader of the humans, while Varian would lead the military campaigns himself, as he likes, let's be honest, he's not much of a diplomat and Anduin could learn more about the Light and make contact with different races of Azeroth.
Shandris would lead the night elves while Malfurion is neutral and Tyrande steps down to take care of the more priestly side of the night elves, both remaining as advisors so to say.
Moira would still be in the council with the other two, but she'd represent the dwarves outside Ironforge while Muradin and Falstad would represent it inside Ironforge.
Gelbin is Gelbin.
Nobundo could finally unite his people, the draenei with the broken ones.
And Aysa? I don't know, the pandaren don't feel much attached as a faction fully to either Horde or Alliance, they're more of a "some adventurers from their lands joined the Horde, some the Alliance".