Have Thrall sacrifice himself to save Grom, and then have Grommash Hellscream take his place in the Horde of Azeroth.
Otherwise, this expansion will have been a completely pointless endeavor, as far as Lore is concerned. And like it or not, Lore IS important to Warcraft, because it's what makes the raid content seem so compelling. Despite having brought so many important a characters back to life, this expansion has been shockingly dull, having done nothing of any value whatsoever with them.
Frankly, the game NEEDS something big to happen. I'm actually not "anti-Thrall" like a lot of people are (well, at least not until the showdown with Garrosh in Nagrand). The character is only hindering the development and expansion of the game's fiction, though. People may have hated Garrosh, but with him as Warchief, it allowed other characters a chance to shine.
Furthermore, Grom has done literally nothing that could be construed as "evil". He waged war, but that is simply part of the Orcish culture -- and unlike Garrosh, Grom has consistently shown a deep sense of honor. Be it as Warchief, Chieftain, General, or even pariah, I think Grom could offer far more for the story than Thrall is clearly capable anymore.
Of course, I'm not holding my breath anymore.