Not going to vote, as I miss the option: "Will play a role but not be the deciding factor".
And btw. he wasn't the deciding factor in MoP either. In Cata, he actually used the Dragonsoul and fired the Killshot on Deathwing. In MoP, he was lying on the ground, defeated, and just got to raise his hammer, AFTER we dealt with Garrosh.
Him defeating Garrosh on Draenor is not a big surprise. As pointed out in "War Crimes", Thrall is a powerful warrior, as well as a VERY powerful shaman, something (in the lore) no mere warrior can deal with...and for all his power and cunning, Garrosh is just a warrior, without the help of an old god, he has zero magical capabilities. And on Draenor, Garrosh will be neither be infused by an Old God, nor shielded from the elements by an Army of Dark Shamans.
My guess is, that at the end of WoD, we will be dealing with a Grommash enslaved and bound by the Burning Legion. That's several steps above Thralls capabilities, so he will be a bystander, even more so than he was in MoP.