Thrall could return, Garrosh was only supposed to be Warchief while Thrall took care of Deathwing. With the Cataclysm over, Thrall is free to put back on Doomhammer's armor, and march right in. Some might say that he was an ineffetive ruler, but the fact is, he kept the peace fairly well with a few exceptions (the various battlegrounds), was the founder of the peaceful side of the Horde. He is a unifying figure among the Horde, and someone the Alliance is at least familiar with.
Nazgrim could also take Garrosh's place. He's probably the highest ranking orc in the Horde regular military. He is a bit fiesty though for the Alliance's liking. He's also a fresh face in many people's minds, which would make him a likely pick. He seems to be one of the younger generation of Horde leaders, Garrosh's generation. He is a strong leader as a general, and would keep the Horde fiercely independant.
Saurfang could also be warchief, he's the overlord of the Kor'kron elite, which is a pretty significant position. He's a warrior, but knows how to deal diplomatically with the Alliance; he lead the Might of Kalimdor against the Qiraji, which was a combination of his Kor'kron and thier Alliance equivalent, the Seventh Legion. This combination was seen again at the Wrathgate, it can be safelly said that though his son was posted there, he was in command of the situation. Saurfang is someone the Alliance knows, and respects to a point. He's a hero of Silithus and Northrend, and would be a leader good for both the Horde and for the Alliance in making sure the Horde's in check. But he's old.
So, though it might be others, who do you think it'll be? Thrall's return? Garrosh's right hand man? Or the hardened veteran?