So who should it be? The obvious choice is one of the racial leaders, though exactly which one is up for debate. I thought it would be fun to analyze some of the potential candidates and tease out what might make them a reasonable choice of warchief both inside and outside the story. Let's start with some of the easily dismissible, for brevity's sake.
This post contains some minor spoilers for patch 5.3, so be warned!
The Snowballs in Hades:
Let's be honest, I don't think these guys have any chance at all, but for the sake of being complete, here they are.
Ji Firepaw: We can scrap this guy right off the bat. He's the pandaren representative to the Horde, but he's a) brand new and b) really doesn't have any reason to care much about the Horde's internal politics at this point. No warchief, this one. You're the weakest link, goodbye!
Now we're into murkier territory - the realm of candidates who, theoretically, could actually have a shot. The rest of the racial leaders are well-developed and well-known, but not all of them are actually well-positioned to claim the seat of the warchief. I'm going to list them in order of least to most likely, in my opinion, with a wild card at the end.
The reason why is obvious: none of them actually like her. Not to put too fine a point on it, but Sylvanas is terrifying. She is ruthless, brutal, and has proven herself willing to use the very same tactics that were used against her to bolster her undead army - the forced reanimation of dead bodies. She claims that, unlike Arthas, she leaves her citizens with their free will intact, however, there is evidence to the contrary. Given her demonstrated tendencies toward brutal totalitarianism (not unlike Garrosh himself, interestingly enough) and gruesome, plague-based warfare, I cannot imagine that many other folks in the Horde - even those who once respected her greatly - actually want her to be in charge. It likely wouldn't be much of an improvement from the current status, just a lateral move. If Sylvanas is acting disinterested, there are some very solid reasons to let that disinterest be, and to not encourage her. For these reasons, I think it highly unlikely that Sylvanas will become the next warchief.
Baine Bloodhoof: Baine has a few solid things going for him. He is Cairne Bloodhoof's son, and Cairne was by nearly all accounts a legendary chieftain of his people. Devastated by years of war with rival centaur clans, Cairne boldly led his people out of their traditional homelands in search of a safer place to settle. He, Thrall, and Vol'jin formed the backbone of the leadership of the New Horde. Cairne also oversaw the tauren's building of Thunder Bluff in Mulgore, forming their first known city. These are huge cultural upheavals, and their successes can largely be attributed to Cairne's astute leadership. Baine comes from a proud lineage, and presumably learned a great deal at his father's side.
Of course, presumption isn't the same as reality, and Baine has made some questionable political decisions recently. Not the same blatantly malicious kind as Garrosh, but the kind borne out of naivete and an unwillingness to face certain problems head-on. Baine seems to struggle with decisiveness. This is not an unexpected or unusual problem for people new to positions of leadership. Baine has a difficult job, and he's learning, but he doesn't quite seem to be there yet. It is my opinion that, for this reason, he would make a poor warchief. I just don't think he has the experience necessary. Interestingly enough, a very similar situation did not stop Thrall from leaving the position to Garrosh in the first place, which is something to think about. And to perhaps fear.
The Best Bets:
These are the folks that, given things in WoW as they currently stand, I believe have the best chance at becoming the next warchief.
Lor'themar Theron: Lor'themar has, to pretty much everyone's utter shock, emerged as a serious sleeper contender here. It's to the point where even Dave Kosak tweeted pictures of "Lor'themar for Warchief" buttons made by Blizzard employees. Given the former prevalence of "Who?" jokes whenever someone invoked the name of the blood elves' leader, I think that if a year ago anyone had predicted Lor'themar would be one of the most dicussed characters of Mists of Pandaria, we would have all had some derisive laughter for them. Indeed, Lor'themar and the blood elves' activity throughout Mists in general is a fascinating plot direction, and it has left Lor'themar better-placed to make a serious move for the position of warchief than anyone ever could have predicted.
Lor'themar does have some attractive skills here that are worth considering. He is, or was, a military commander first and foremost. As Sylvanas' second-in-command prior to and during the Third War, he has experience leading armies through combat, and that kind of martial prowess is clearly an important component of Horde leadership. In addition, Lor'themar has also now spent a number of years heading the rebuilding of Quel'Thalas, both as a nation and as a culture, and that road has been extremely fraught. Undoubtedly he has learned a great deal about political leadership during this time as well, and this particularly is where he shines over someone like Baine, who is still struggling with those things due to the newness of his position. Lor'themar went through a similar crucible throughout and immediately after Burning Crusade, and has emerged on the other side of it tougher and savvier.
Vol'jin: The more information we see coming out of patch 5.3, the more my money is sliding toward Vol'jin as the next warchief. Despite Lor'themar being the one to take the first concrete steps toward open rebellion against Garrosh, Vol'jin is the one taking the rebellion's reigns on its home turf. For an even further meta-analysis, Vol'jin is also the subject of the latest WoW novel, and if that isn't a big flashing sign that he's about to be a Very Important Character, then I don't know what is.
Vol'jin has been awaiting his day in the sun for a long time now. He's one of the more-established WoW racial leaders, having been first introduced in Warcraft III, but he didn't have much of a spotlight in WoW itself until relatively recently. The retaking of the Echo Isles back in Wrath was the first time I remember him being an active mover and shaker. Since then, his roll in the game has slowly garnered more spotlight, and seems to be culminating in this rebellion. Unlike Lor'themar, whose in-game development came rather rapidly and somewhat unexpectedly, Vol'jin's has been building for a while now. He's even fulfilling an old promise he made. There's a certain narrative poetry to Vol'jin leading the uprising and taking Garrosh's place.
As for Vol'jin's credentials, well, he certainly has experience leading a people. But does he have experience leading a nation? That, I think, is one of the crucial differences between Vol'jin and Lor'themar. Silvermoon was once an enormous, thriving city, and Quel'Thalas was once a powerful nation. Though Lor'themar has inherited only its remnants, his perception of leadership is surely filtered through what it means to lead an entire country. Vol'jin doesn't have that particular vision - or does he? It's a bit hard to say, but this is the one sticking point I keep coming back to with regard to Vol'jin's leadership. The jump from leading the Darkspear trolls to leading the entire international confederation that is the Horde could prove to be an extremely difficult one to make.
The Wild Card
Thrall: It's possible. Thrall was warchief when WoW began, I can see the appeal of him being warchief once more. After an extremely active presence in Cataclysm, he's taken a serious back seat during Mists, though he hasn't disappeared entirely. We know he tells Vol'jin that he just can't believe that every orc in Orgrimmar would support Garrosh's actions, and it seems that he's going to take some kind of role in the rebellion, though we don't know exactly what or how.
Do I honestly think Thrall will be reinstated as warchief? I'm really not sure. Prior to Mists of Pandaria, I would have said yes, but ongoing developments have me actively doubting my interpretations and questioning my assumptions in a number of ways. I like that. I'm excited to see where it will go, and to discover who will, in fact, be the next warchief of the Horde.