No idea how things will turn out, though a role reversal could be interesting. Thrall's not so prideful he'd be 'above' taking orders from someone I think.
And it's all speculation. I'm guessing it'll be Vol'jin just on the wording Wrathion uses, but he could just be purposefully vague. I'm also not counting Lor'themar out completely but think he's the least likely of the three to get it. I'd say I"m 65% on Vol'jin, 30% Thrall and 5% Lor'themar. But again all just me speculating. We'll find out when we get the cinematic.