Ya, I've kind been thinking Vol'jin would try to unite the troll tribes. I hope so anyway. that would be badass.I would like to see Vol'jin as Warchief. However, I think their are greater plans for him. Incoming long post, sorry, but I've put thought into this.
Looking at recent Troll lore (Rise of the Zandalari onward) we see that the Zandalari Prophet Zul/Rastakhan has had a vision of a united Troll Empire, once again. However, of the Troll leaders shown Vol'Jin alone defys the will of the Zandalari.
Instead of just settling back as leader of the Darkspear tribe and letting the Zandalari and other Trolls unite he plots to overthrow them. Working with the Humans, Booty Bay Goblins, Nessingwary Jr.'s hunters, and players, he pushes the Gurubashi out od Stranglethorn and back into Zul'Garub where their leaders are defeated. In the Ghostlands he unites the Farstriders and Silver Covenant against the Amani, and with Player help attacks Zul'Aman defeating their leaders.
The Drakari of Zul'Dark havce sacraficed their Loa and have largly been defeated by Players. It was also implied that the Sandfury of Zul'Farak have been defeated. Except for the Zandalari the remaining troll clans either have strong Horde ties (Raventusk), are all but exterminated (Mossflayer), or Players have been attacking them for years.
It's been hinted that we will be attacking the Zandalari directly soon. Vol'Jin will most likely be the leader of this assault. The end will have to be the death of the Prophet Zul and Rastakhan, and probably the death (again) of Hakarr. Since the Raventusk tribe is friendly with the Horde, it would be logical for them to assist in the defeat of the Zandalari.
But what would happen ofter the defeat of the Zandalari? Typically in warlike cultures, like the Trolls, when you defeat a group you gain everything that was theirs, or kill off the entire group. Vol'Jin doesn't seem like the type to exterminate an entire group without reason, there are many civilians in ZG and ZA (yellow mobs). This would mean if Vol'Jin was leading the attack on the Zandalari, in effect he could become the next Rastakhan.
With the Zandalari effort to unite the tribes (and at least the Drakarri following them) this would put Vol'Jin as the leader of a new troll Empire. So Vol'Jin will be too busy leading theTrolls into a Golden Age (fulfilling Zul's vision), as possible allies with the Horde, and using the combine might and knowledge of the Trolls to help overthrow Garrosh.
tl;dr Vol'Jin won't be Warchief, instead he will be the Rastakhan of the Troll Empire.