In 5.4, Garrosh Hellscream will be deposed, and a new warchief chosen to lead the Horde. Who do you think the next warchief will be?
I think that Blizzard will not pick someone obscure. They will pick someone with a lot of name recognition, who makes sense to the average, casual Horde player. With that in mind, here are my rankings:
Front-runner - Vol'jin
Vol'jin has gotten a lot of screen time, including a full novel. He's also the driving force behind the rebellion, and has been a racial leader for all of WoW. That makes him the obvious choice.
Contender - Lor'themar
Lor'themar has also gotten a lot of screen time in MoP, and has been developed into an admirable, competent leader. However, the big negative is that he is a blood elf. The Horde's motif in Warcraft is to be the "monstrous" faction, the outsiders. Having a pretty-boy blood elf be the Warchief cuts against that, and cuts against the Horde's identity in a major way.
Contender - Thrall
Thrall was the previous Warchief, and he has very high name recognition. But he's also the one who left the Horde, and it might seem weird to just waltz back into leadership. However, the orcs will still need a racial leader. Thrall is pretty much the most important orc left, and is likely to be the orc racial leader. Having Thrall be leader of the orcs--but not Warchief--may seem odd.
Dark Horse - Sylvannas
It's Sylvannas. Massive name recognition and popularity, and cunning enough that her ascension could seem plausible. However, she's been on the sidelines for most of Mists, so it would be odd for her to come out of nowhere and grab the throne. Plus, what would she really do as Warchief? She has her kingdom and her armies over in the Eastern Kingdoms. Taking on leadership of the entire Horde seems like it would gain her a lot of trouble and not many advantages.
I would expect her to use this opportunity to throw out the Kor'koron Overseers in the Undercity. That seems like a very Sylvannas-like move to me.
That's how I see the race for Warchief. I think Blizzard is going to pick Vol'jin. He's the one they've been setting up, the one who's story arc leads to the mantle of Warchief. To be honest, picking anyone else really seems like deliberately making a twist ending, just for the sake of having a twist. That rarely works out well.