To me, it seems that Lor'themar Theron would be the obvious choice. He could easily gain public support by denouncing Kael'Thas and removing Grand Magister Rommath from power, replacing him with a mage loyal to himself. Getting support from Halduron Brightwing shouldn't be too difficult to recruit as he could probably offer him some favor to make the war against the scourge easier.
This is where I've become uncertain. In the current lore, do Deatholme and Stratholme still stand, or where they destroyed? Are they just leaderless but still filled with the undead, or are they pretty much cleared out? In my reasoning, he could march on the Eastern Plaguelands and smash the scourge against the Forsaken. Creating a strong alliance with Lady Sylvanas should be easy because she is a Blood Elf. Also, pushing the Alliance back into the Hinterlands/Alterac Valley (not sure where that exactly comes out, I play Horde).
The Blood Elf/Forsaken alliance could then work the Darkspeare and Tauren to remove Garrosh from power and create an equal balance of power among the Horde instead of the current leader being the Warcheif.