Won't be him, imo. There's no evidence to show he really cares about the other Horde races (yet); he had a choice between letting his jaded people fend for themselves or having the bulk of the Horde as protection. His deal is the protection of the Blood Elves and nothing else, so far, though contrastingly, it was him who turned up in Pandaria with Garrosh, and he does seem to have some rush of faction-pride every now and then ("Brothers and sisters of the Horde!"). From a military standpoint, he is capable, probably with decades or even centuries of experience. He certainly has the attitude, too (and it's remarkable how he's developed since Shadow of the Sun), charging into fights with the intention of splattering the blood of his enemies over his sword. Has he got what it takes to be Warchief, however? Both yes and no, I suppose. Neither has he had a model upgrade, which sounds like a stupid reason to say he won't be Warchief as Garrosh was upgraded when he took the mantle, but Lor'themar appears in the 5.2 trailer with his current model which kind of reaffirms that he'll be stuck with it unless something happens to alter his appearance.
I think the reasoning behind his development is purely that he desperately needed some. Having him lead the Horde assault against the Thunder King was a good way to do it. He's now a strong and likeable character, and like Kosak said, the average player can get a good idea as to what his deal is. Yes, he's also a good candidate for a potential Warchief, and though I don't think it's likely he'll take the title, it's hard to say anything certain until we know the rest of the story.