Vol'jin - "used" to be suitable. But he raised rebellion against the Warchief despite his oath. Yeah - its a pity they brought it in, yeah it was "cool" at the time but there it is. Others may have followed...but HE led. I personally don't think he came out of 5.3 that well. But that's just me. Overall, however - I think he si the front runner with his major flaw bieng the relative size of his Tribe. As a rising star, however - maybe he can get one of the BIG Troll tribes/nations to back him.
Gallywix? Totally unsuited. He'd sell the Horde. No interest whatsoever.
Ji? Yeah....a non-entitiy.
Baine? Young and inexperienced. He also appears to be respected, is liked by the Alliance and has Cairnes legacy as co-ruler of the Horde to back him up. He'd be suitable but he'd need advisors and as with Vol'jin it partially would depend on how the Orcs see him for violating the Oath.
That leaves the Orcs racial leader as a candidate. Saurfang and Thrall would both seem to be good choices.
Other candidates? Rexxar. Untouched by the Civil War, yet respected by both sides. A potential.
A Council? Unlikely given issues with the Forsaken and so on.
Doing away with the Warchief? Downgrading it to a merely military title akin to the Alliances Supreme Commander? Again an idea.
An Orcs who fought for Garrosh - out of honor? Nazgrim would have been a good choice under this criteria.
So...Vol'jin. Baine. Rexxar. Saurfang. Thrall. Anyone else?
Personally - what I'd like to see the next Xpac focus on is rebinding the Horde together. To actually unify them. They've had three civil wars in three years. Not good. That's also not counting in the BElfs against Kael, nor the various splits in the Orcish Horde.
Right now, the Horde doesn't seem able to provide the protection the Forsaken want and need against the Alliance. Gallywix doesn't care so long as he gets paid...but as long as he is with the Horde, he's esentially locked out of the Alliance markets. LT will have to decide whether staying in a dsinitegrating Horde is inded best for his people. Ji came to the horde to help out - and found that he wasn't treated that well and the Horde essentially stood for everything he opposed. And since many of the Orcs sided with Garrosh, there is going to be a certain element of distruct against one of the Hordes more powerful members.
Of copusre, all this is based on the characters right now. If Blizzard wanted Varian to be Warchief, that'd be the case.