As for the Forsaken, although Nathanos would have been an excellent, logical choice to train the new legion of Forsaken hunters were he still alive (and undead), they aren't without a teacher. Sylvanas Windrunner was, after all, the Ranger General of Silvermoon -- and after she regained her body she became the first dark ranger. According to the RPG books, dark rangers aren't quite like the hunters and rangers we know in WoW -- rangers themselves are highly attuned with nature, and death severs that connection, so the dark rangers turn to shadow magic rather than the natural magic of the wild in order to accomplish what they used to do.
So we have a few working theories about the forsaken hunters -- Nathanos may have trained a few individuals during his time as Champion of the Banshee Queen, or perhaps the dark rangers have started taking non-elven undead under their wings. There's also a third, much more grim possibility, and it lies with another organization that has roots in the former kingdom of Lordaeron -- the Scarlet Crusade.
The three dreadlords worked with Sylvanas and helped her regain her body and attempt her revenge on Arthas -- but what they did not expect was that Sylvanas would then turn on them. She persuaded Varimathras to join her side, had him kill Detheroc as a show of his allegiance, and then attacked Balnazzar's base, where Varimathras "killed" Balnazzar when he would not join sides with the Forsaken. Balnazzar escaped, but he swore his vengeance. When the Scarlet Crusade invaded Stratholme, he possessed their leader in order to enact that vengeance.
It didn't take much to push the overly zealous members of the Silver Hand to form the Scarlet Crusade -- an organization devoted to wiping out the undead. And it took even less pushing to cause the organization, formed under what was though to be a noble cause, into performing unspeakable acts in the name of "justice." The Scarlet Crusade turned from an organization devoted to destroying the undead, to an organization that killed anyone and anything on sight -- after all, if a lone human were wandering the forests of the Plaguelands, it could be and should be assumed he was in actuality an undead, just freshly raised.
Over the course of World of Warcraft, the Scarlet Crusade has been finding themselves slowly losing their war, and by the time Cataclysm comes into play, the Scarlet Crusade is a shell of its former self. Few members remain, and those that do remain are quickly dying out. The Forsaken may have taken this opportunity to raise several of these dead former Scarlet Crusade hunters and used them to train hunters of their own. A grim fate for those once fervently seeking to eliminate all undead -- to become the very thing you spent your life trying to destroy. But as we've seen from many of the spoilers surrounding Cataclysm -- it's a grim expansion.
As for the human race, and their sudden interest in the hunter class -- one thing to keep in mind with the humans of Stormwind is that they've had a fairly shaky history as of late. The kingdom of Stormwind was utterly destroyed, and then rebuilt -- it could very well be that there simply was greater need of other tasks than hunting. Or, given the dissolution of the Scarlet Crusade, it could be that some former members have left the ranks and joined the Alliance whole-hearted, and agreed to teach others in the ways of the hunter as a sort of penance for the atrocities they committed while serving the Scarlet Crusade.
Regardless, the hunter class is relatively easy for roleplayers and storytellers to justify if needed -- the class choice isn't quite as far of a stretch as previously thought, unlike other race and class combinations that have left people mystified. Combinations like the noble tauren, and their sudden interest in paladins and priests. Next week, we'll take a look at the "holy cows" and how they fit, along with a dive into tauren history -- or what exists of it.