There used to be a ton of speculation about hero classes; people always love to speculate on things about which there is scant information. They were to be loosely based upon lore, previous WC games, and things you see from the game itself. Let's see if I get this straight...
For each class, there would be three possible hero classes. Each would follow a certain talent tree further. For example, if you were a Paladin who became a "Mountain King" (I think that was one of the ideas thrown around), you would gain an extended Protection tree. It would basically add those extra two tiers to your current tree, which you could then spend points in. I remember that there was also a bit of variance based on race, too. For example, Troll Mages would become Hydromancers (Frost), while Gnomes would be Archmages (Arcane). Humans would be either Archmages or Pyromancers. I was upset when this idea was out, because as a gnome mage, arcane was my least favorite tree :-P
I'm trying to remember other class/race combos...
Shadow Priests became Necromancers... maybe Warlocks, too?
Warlocks could become Blood Mages (this raised problems, because first of all, a blood mage ain't a warlock, it's a mage, and second of all, mages felt this stepped way too much into their domain).
Rogues could be Assassins, and a couple of others (they had really cool names, it's too bad I don't remember them).
Paladins were, I think, Mountain Kings (Protection), Templars (Holy), or Crusaders (Retribution). I had a Paladin and a Mage, so I paid most attention to those classes for info.
I think Druids would be something like Archdruids (Balance), Druid of the Claw (Feral), or Keepers of the Grove (Restoration)
OH! And I think one of the rumors said that depending on the race/class combo, you might end up with different classes as the same hero class. Like a rogue could become a Demon Hunter, but so could a hunter.
There were other hero classes, like Blademaster (Orc warriors), a bunch of sinister offshoots for rogues, and many other ideas. But that was the most coherent rumor that I heard.
It seemed like a fun idea, but like all fun ideas (Worgen from Gilneas, restored Gnomeregan, Ashbringer from a few months ago, etc), they don't amount to anything until Blizzard acts upon them.