I do want to say right off the bat that I personally don't think any of the folks discussed in this post really have a chance, but more unexpected things have happened in the course of WoW's plot development!
Varok Saurfang and/or Eitrigg: Saurfang and Eitrigg were names mentioned often in the comments of the previous post, and I understand why. They are both established, well-liked orcish characters. Saurfang's son, Dranosh, was an important figure in Wrath before he was killed. Unfortunately, as outlined in The Shattering (pg 97):
Both Varok Saurfang and Eitrigg were considered once before and it was concluded that they were too old, and in Saurfang's case, too mired in grief, to be effective leaders. Several years have passed since then and neither are any younger, though it is possible Saurfang is now in a better place emotionally. Ultimately the major issue I see here is a meta-problem: neither of them have been active in the current events of Mists of Pandaria. By contrast, take Lor'themar, whom Blizzard deliberately positioned throughout two content patches in order to to bring his character up to speed prior to the formation of the full rebellion. If Saurfang or Eitrigg were truly going to feature prominently, why weren't they given the same consideration?
The perfect choice was one [Thrall] could not have - Saurfang the Younger. Youthful, charismatic, and yet wise beyond his years, he had been the brightest star in the sky of Horde warriors before the Lich King had slain him. His father, though not quite broken, had been emotionally devastated by the recent events. Too, the orc was too hold, as was Cairne, as was the deeply trusted Eitrigg.
Warlord Zaela: Zaela's name came up as well, and to my shame, I must admit that I had not thought of her. I personally would love to see Zaela at the Horde's helm. However, there are some worrying datamined files that seem to show her as allied with Garrosh throughout 5.3. Personally, I hope beyond hope that she is working to undo him from within, rather than being a Garrosh loyalist. It's the only situation in which she could realistically be a candidate for warchief; otherwise she'll likely end up as boss fodder in an upcoming raid.
Assuming Zaela is not actually on Garrosh's side, she's got a lot going for her as a potential warchief. She's smart, cunning, tough, and ferocious. She has experience leading a people, though, like Vol'jin, perhaps not leading a nation. Still, I think she's a wonderful dark horse candidate. Zaela 2013!
Something entirely different: Another idea I've seen come up a lot is that there will be no single replacement for Garrosh, and the position of warchief will become a war council, instead. This would be a very interesting story direction, and a good metaphor for the true dawning of a new age. If I were to design my own ideal war council of the Horde, I would choose Vol'jin, Baine, Lor'themar, and Zaela for its principal members, but let's face it, there's no way Sylvanas would let herself be left out like that, so I'll include her too. Folks like Thrall, Eitrigg, Aggra, and Saurfang would likely have strong advisory positions, and potentially even Ji Firepaw, but I can see Gallywix being somewhat shouldered out - no one seriously likes that dude. Maybe Sassy Hardwrench could be the goblin advisor, she's a great character.
The case of the next warchief is one of the great current plot mysteries of WoW, so let me know if there's anyone else I've missed!