I think it makes sense, cause he helped orgrimmar out once and knows thrall, too. He is half orc, half ogre and a powerful hero. After Thrall and Garrosh, you can easily say Saurfang could take over, but i think he is too tired after all the wars and his son dead, and old.
Rexxar could really fit in here as a new warchief of a new horde. when its needed another race instead of an orc should be warchief why not take Rexxar, who is connected to Thrall and Orgrimmar and is just half ogre half orc. I think, a troll or tauren as Warchief, is pushing the borders, orc citizens might moan about it.