Well, it would seem I was right about this. That's one for two! Now I just have to make one more correct prediction, then I can title a post "two out of three ain't bad" and check that one off my bucket list.
The xpac will be Cataclysmic, Worgen will join the Alliance, and Goblins, the Horde.
Being a giant geek, I immediately began to wonder what classes those races might include. Death Knights are a foregone conclusion, and I doubt we'll see a new hero class this xpac. If I were Blizzard, I'd also be favoring healing classes and trying to avoid plate dps classes where possible. It would also be nice to give alliance another shaman race and horde another paladin race.
But then, we have to consider the lore. I know, "lorelol". And that's a problem. We can speculate all we want about the lore, but in the end Blizzard can always pull some crazy retcon like they did with Draenei, or with BE Paladins, and just say "well this is a previously undiscovered branch of the Goblins that really loves the earth and can be a Shaman, despite decades of Goblin deforestation and stripmining" or whatever. Nonetheless, I think they'd prefer to stick to lore where they can, and only deviate when they would consider it a necessity for gameplay and balance. With that assumption, I went and did some research on the two races.
First, we've got the Goblins. We know a lot more about them, being an established race and all. The ability to have neutral goblin towns is so useful to the developers that I think it's safe to say that we won't see the entirety of the Goblin race joining the Horde. Most likely it will be one of the trade princes deciding to break off from the rest (Goblin society is capitalism-focused, and ruled by a Coalition of trade princes. We've only really met goblins from two of these cartels; the others haven't really appeared in Kalimdor or the Eastern Kingdoms). Steamwheedle Cartel and the Venture Company, for instance, will likely stay neutral or hostile to both factions. This opens up a nice opportunity to revamp the character models for player Goblins as well, just as playable Darkspear trolls look different from NPC forest trolls.
According to wowwiki: "Officially, five trade princes exist, though one or two may be lurking about in the shadows." The leader in charge of the Venture Company is not technically a trade prince, but a lesser, independent "Mogul", so it's possible that the splinter of goblins that become playable are under one of them instead of a prince. On the other hand, thee Steamwheedle Cartel was the group that originally sided with the Horde, so it's possible they will again, or that this is a splinter group that separates from the cartel.
It also means that these goblins can be a little different from the ones we have encountered so far. I brought up the 'ole wowwiki to get some background on the race, and learned a lot. For instance, the goblins claim that "if not for their frail physical forms, goblins would rule the world", given their high intelligence and technology. This gently points away from warriors, but we might be meeting a cartel that bred 'em a little sturdier, or might easily find that goblins, like gnomes, can make fine warriors. Druids and shaman are pretty much a no go with goblins, and would take a massively insulting slight-of-hand to pull off given their history of complete contempt for mother nature when it stands in the way of profits. Traditionally, they also tend to focus on material pursuits and find magic and religion frivolous, meaning that they don't tend to be priests, paladins, mages, or warlocks naturally ("Goblins place their faith in themselves and in gold. They raise eyebrows at insubstantial concepts such as shamanism and the Holy Light, preferring gods they can see, weigh, and spend"). I think this is a much easier hole to plug: just say this particular cartel specialized in the trade of magical artifacts, and over time some particularly gifted members learned to be mages or warlocks. That would fly just fine. Or maybe this happens to be the only cartel with religious beliefs, and maybe make that the reason only this one cartel decided to break off and join the horde. Then you could at least have priests.
Goblins, with their interest in gold and firearms, will almost certainly be able to play rogues and hunters (this will be awesome, with pets they could almost ride!).
Meanwhile, we know a whole lot less about the Worgen. I think it's clear that the Worgen faction that joins the alliance will not be the same worgen we have encountered in Duskwood or while dealing with Arugal. Those feral worgen were summoned from another dimension (much like Outlands) where they battle a relentless enemy known as the Emerald Flame, which I'm gonna go out on a limb and say is the Burning Legion (since that dovetails nicely into the idea that the Naaru are using Azeroth to gather races from many different planets to take a final stand against the Legion). Arugal summoned them with magic, while a Night Elf summoned and commanded them via an artifact known as the Scythe of Elune. Supposedly, the Scythe appeared in response to a prayer to Elune for help, but quickly spiraled out of control, summoning Worgen all on its own. It is believed the Scythe was lost in Duskwood (and is the source of the darkness there), but may have been found by someone (perhaps the LotR ripoff "Black Riders" that were searching for it). So one viable avenue to bring them into the world is to have them flee to Azeroth to escape the Legion, and pull something like they did with Draenei where the Worgen we play are a more intelligent, "higher" version of the race. Perhaps the ones we pulled through previously were warped by the summoning, or were some lesser slave race akin to apes. If Blizzard goes this route, it opens up almost infinite class possibilities, since we know almost nothing about their society.
On the other hand, and far more likely, the Worgen we play have already been right here on Azeroth all along. If you've ever played an Undead character past level 10, you've probably been to Silverpine forest, just southwest of Undercity. In the southern end of that zone lies Arugal's Shadowfang Keep, and the small town of people he magicked into werewolves. If you continue south, you'll find an impassable wall: the Greymane wall.
Beyond that wall lies the human nation of Gilneas, sealed off completely from the rest of the world since before the Third War. No one has heard from them since. Their king, Gen Greymane, was isolationist and basically considered the problems of the Alliance to not be his problems. The position of this city on the seaboard, its proximity to SFK, and its previous association with the alliance and lore importance, and the fact that it is an already-existing capital city for the developers to use, all point to the possibility that playable worgen will in fact be Gilneans, somehow transformed into Worgen-like creatures during their isolation. This would explain why the Worgen look and think differently from the ones we have encountered so far, perhaps being more like human/animal hybrids. So much about this one fits that I find it to be highly likely. This would create a ready-made capital city on that continent, while the goblins could easily settle a trade city in the open lands of Kalimdor. The goblin race is based in Kezan, a large island in the South Seas, of which wowwiki says "Several cities exist on Kezan, including the nefarious Undermine," which will likely be the neutral city (the "Dalaran") of the next xpac, making it an unlikely, but potential, capital city for the playable goblins. One of the other "several" cities might be the starting area, with the early leveling zones being the surface of Kezan and the passage Undermine blocked off until a certain level.
Another nice supporting tidbit from wowwiki: "After Patch 3.1.0, the loading screen/ship route from Menethil to Theramore no longer crosses Gilneas." I think we have a winner!
I know that was a little off track, but the fallout is that we'll either have a situation where (a) the devs are free to make the worgen anything they want, or (b) the worgen are an offshoot of humanity, and share their culture and general learnings. Since it's useless to speculate about (a), let's consider (b).
It would make warriors, rogues, mages, and warlocks obvious choices. Druids would be right out, since a furry turning into a furry isn't that exciting, and the race has no connection with the elves anyway. Hunters also seem likely, since the peninsula of Gilneas is much larger than the city itself, and being isolated probably drove them to hunt to survive (plus keen animal sense help with the hunt). However, I would be able to buy the idea that the worgen develop a special kinship with, and begin to learn Shamanism from, the draenei, being the other "freaks" of the Alliance (and perhaps the animal transformation lends these humans a closer relationship with nature). Unfortunately, it's a stretch.
So let's combine the game design parameters with the lore and see if we can come up with educated guesses about what classes each race might have.
Looking at the population numbers, you'll find that currently paladins are tied with death knights as the most popular class. As a dev, I'd avoid giving either race paladins. I doubt they would bar those new races from being death knights, though it is possible. I'd want to favor healing classes and disfavor plate dps classes without losing all tank options. Druids don't really work for these races, and I'm avoiding paladins, so DKs and warriors for both seem obvious to give them tanks. Without druids or paladins, I'd try to do whatever I could to give both races priests and shaman. However, shaman don't work for goblins without insane lore twisting. Hunters are popular, so we should only use that class if we need to fill in some space, and also consider that right now we've got 3 alliance hunter races and 4 horde hunter races. Rogues fit perfectly for both classes, and along with warlocks and shaman are the least-played class in the game. So fit in rogue, warlock, and shaman where I can.
Where does that leave us?
(Hunter or Warrior)
So we avoid the nature-lovers and focus on the intelligence of the goblins. If Blizzard wishes to avoid the arcane, we may see both hunters and warriors, but not mages (though the goblinish affinity for fire and explosions are in favor of it).
(Hunter or Shaman)
EDIT: I neglected the 6-class limit in my original post, listing both hunter and shaman. I've changed it to an either/or situation.
Here we mimic the human classes and add shaman if at all possible. The biggest leap I take here is dropping mages. This is partly because the Worgen already have too many options that fit better, and partly because mages are quite popular. I also find it likely that the combination of no ties to the Kirin Tor along with the worgen transformation probably make arcane practitioners quite rare in Gilneas, a nation known more for its burliness than its cleverness.
So there are my predictions for which classes will be offered as options for the new races. There are far too many variables for me to be exactly right, but if I at least had the right idea I'll probably count it as 2 out of 3. :)
UPDATE 8/31/09: Well, though my reasoning turned out to be mostly sound, my conclusions were off-base here, mostly due to information that I couldn't have had or guessed, such as the expanded class selection for the other races. I'm much happier with what they chose to do compared to what I speculated, so I consider myself lucky to be wrong on this one. Now excuse me while I beat up the guy who sold me this crystal ball.