Sorry for putting the title as if it was a fact and as if I had inside info, but that is where the story seem to be heading to.
I've some points I would like you to consider (Excuse my english, it's not my native tongue)
1) He has been getting the lore spotlight for quite some time now. Garrosh obviously thought he was the most dangerous Horde leader out there and that's why he plotted to kill him. Then Vol'Jin went into hiding and plotted to depose Garrosh and save the Horde he knew and loved. He has been the one that has been doing most of the thinking and muscle job in the rebellion. They are building him to be the Savior, so to speak. And he has the support of the majority of the Horde's leaders, even Thrall. This is particularly important in rebuilding Orc relationships with the rest of the world, as Thrall is regarded in high esteem by most folk. The fact that Thrall sort of green-lighted Vol'Jin and openly supports him means, in my opinion, that he is seeking to redeem the mistake he made when he gave the Warchief mantle to Garrosh. He wouldn't risk supporting a person he didn't see fit to lead the Horde, because his mistake with Garrosh has brought too much pain, war and death.
2) After this war, and Garrosh's racism (and therefore a lot of Orc racism vs other Horde races), Orcs are not going be as important as they as been in the Horde. I still think they will be the biggest, most numerous driving force behind it, but in much less emphasis than pre-Garrosh crisis. Orcs numbers will dwindle, as well as that of the rest of the Horde races involved in the civil war. Who has the best possibilities of recruiting new allies to bolster the Horde? Vol'Jin and the Trolls. There are Trolls in every corner of Azeroth. They are powerful, yet divided. After the Zandalar War is over and the thoughts of a new Troll Empire are put to the ground, the closest thing that the trolls could get that could resemble that new Troll Empire would be the Horde. With Vol'Jin at the helm, I wouldn't be surprised if the Zanladar and the rulers of each particular non-Darkspear troll tribe would want to, at least, parley with him...and ask him to recruit warriors from each tribe so they could prove their might and all that (Trolls are prideful and full of ego, after all). At the end of the day all the Zandalar want is to save the troll race from extinction. Obviously fighting the Horde and the Alliance is not going to work. Their best bet to achieve that goal would be to fix their relationships with Vol'Jin's Horde.
3) Vol'Jin has worked with the Alliance in the past to help face the Zandalari threat. This is mean he is a known face in the Alliance and he can be diplomatic; the opposite of Garrosh. And now that I think off, I dont think Vol'Jin is hated by anyone in the Alliance lore-wise.
4) Time for a non-Orc Warchief, yay or nay? I'd say Yes. And which race is the closest to the original Horde, built mostly upon Orcish traditions? Forsaken? Not by a mile. Blood Elves are too sophisticated and beautiful for a "Blood and Thunder Horde"? Goblins just care about the coin. That leaves you with Trolls and Tauren. Two warrior races with strong ties to nature. But since Vol'Jin has the spotlight, my safest bet would be for a Troll.
5) He's got a new book coming out and he got a model rework. While that is not proof of anything and correlation does not imply causation, that ought to count for something.
TL : DR version
1) Vol'Jin is being presented as the Savior of the Horde.
2) Vol'Jin can help replace all the casualties of this civil war accepting the aid of the rest of the Troll tribes.
3) Vol'Jin has proven to be diplomatic.
4) A non-Orc warchief would be refreshing, and Trolls seem to be the go-to guys.
5) He got some love.