My vote i think will go to vol'jin; reason being he is cunning, intelligent, wiity, and above all he desires peace. Amongst the leaders of the horde at the current time i think only a couple of other leaders/generals fit the billing for warchief, and i believe he would be accepted as a suitable replacement. Lets face it... i don't even know the name of the blood elf leader... because he never makes appearances, and is not even remotely important to the lore. Sylvanas while cunning, does not want peace, and i believe the trolls/orcs/tauren would not let a forasken lead them.
Baine bloodhoof is another possibility, although his youth in my opinion might be his greatest detriment. From what i have read he is almost exactly like his father, except he is willing to use more aggressive actions. He is spiritual, and respected amongst the tauren. I think the majority of the horde would accept his leadership.
Couple of other randoms;