Thrall's heir couldn't be made Warchief because at the moment, Thrall has no claim to the leadership of the Horde--he abandoned it in its entirety so that he could focus on being a shaman ("you can be Warchief or World Shaman, you cannot be both," and all that).

That said, Thrall has strong ties to the current leadership of the Horde--most would like him to take command over Garrosh, so it wouldn't surprise me to see him make it back in one day... but as for orc loyalties...
As it stands now, Garrosh is active Warchief. And seeing how loyal the orc side of the Horde is to Garrosh now, it wouldn't surprise me that for Thrall to come back, and him expecting to take back the reins with no hassle, that he would then have to take it back by force to prevent Garrosh from doing something stupid (probably with the help of Vol'jin).

At that point, when and if Thrall takes up the mantle of Warchief again and marginalizes his duty as a shaman again, he can pass the Warchief mantle to his son just as he had to Garrosh previously.