I think Thrall will die. He is indeed the strongest Shaman alive but going alone to Orgrimmar is risky even for someone like him. He will kick ass but I think with some luck Garrosh and his army will kill him then finally shit hits and fan and the entire Horde + Alliance will go against Garrosh and his army. Hearing Thrall audio made me think that indeed such thing will happen and Vol'jin will be a temporary Warchief until Thrall's child comes to age then he will pass his title to Thrall child.