First let me say a word on why I think there will be new races in this expansion and why it will be goblins and worgen. First of all, the Maelstrom and Emerald Dream both seem like logical places to go next, and they both have connections with one another that could tie together well in an expansion story. You can read Lorecrafted for a more detailed speculation as to how the lore elements could fit together -- I doubt it will be exactly like any of us predict, but many of these same elements will likely be present in different ways.
Secondly, if you look at the new Halloween Masks of the goblins and the worgen (pictured above), you can see that they actually have new model textures in them. The goblins pictured there are not just the same goblin faces we see in the game today, but totally new ones with more detail and nuances. The same is true of the worgen -- you can see that they don't look like the ferocious worgen we see in the game now, but rather like noble warriors of some sort, much more dog-like than the savage wolf-men we see now. These are player-character quality textures, likely for entirely new goblin and worgen models which will be released with the new expansion. Besides, it just makes sense that after getting a new class in the last expansion, we'll get new races in this one (though admittedly I would prefer to get both!).
Then again, it's entirely possible that all this is just our own idle imaginations, and that the actual expansion will be entirely different! Looking forward to the future always has that problem. Sometimes you'll be right, and sometimes you'll be wrong.
So how well do the worgen fit as a potential player race? Obviously there are many things we don't know about them, and many possibilities. We know for sure that the worgen we play will not be the same type of worgen we've been fighting all along. Some theorize that they could be former humans of Gilneas who have turned into worgen by some magic, but I think this is unlikely. My theory (which some others have thought of as well) is that they are creatures originally from the Emerald Dream itself, the majority of which have gone mad through their battle with the "Lords of the Emerald Flame" which I suspect has something to do with the Emerald Nightmare. A small remaining bastion, the one which will join the Alliance, still maintains the good fight, and desperately turns to others for help, much like the draenei did in the Burning Crusade.
This fits with the story of Velinde Starsong, the night elf who originally summoned the worgen to Azeroth for the first time, using a magical gift from Elune herself, the "Scythe of Elune." If, over time, the Nightmare corrupted the worgen, then they would indeed appear as they are in the game now: vicious and evil to the core. Perhaps at some point we will catch up to Velinde Starsong, and see what happened to her after she disappeared.
In any case, whatever the story behind the worgen may be, they could be the Alliance's first "tribal" culture, similar in some ways to the orcs, the tauren, or even the trolls. They're sure to be different, of course, perhaps more like the night elves, and possibly following the same religion as well. Together, the worgen and the night elves would provide a counter-balance to the current dominance of Light-based beliefs which the humans, dwarves and draenei follow.
Worgen would also be the Alliance's first beast-like race, in this way a sort of counterpart to the Horde's tauren, though in size and stature they would probably be more like the trolls. A roleplayer with a worgen character could explore themes about the beast within, about struggling to keep your sanity against forces which could easily turn you into a monster you abhor.
The goblins have a city in the South Seas, very near to the Maelstrom. They are a much easier case, since they have been almost on the Horde's side for so long now. But it's important to remember that it definitely won't be so simple as just a group of goblins deciding to team up with their orcish buddies. The goblins that may end up joining the Horde will not just be regular goblins like all the others. They may be an outcast trading company of some sort, no longer part of the system created by the trade princes of Undermine. Whatever reason they may have for abandoning their formal neutrality, there's sure to be a story behind it, probably one which will draw us closer into the Maelstrom, the Emerald Dream, or both.
For roleplayers, goblins would give the Horde a more cutthroat counterpart to the gnomes, much as blood elves gave them a counterpart to night elves and humans. Many roleplayers have long enjoyed the inventive curiosity of the gnomes in their scientific experiments, and if goblins join on the Horde side, there will be even more madly scientific fun there as well.
In addition, if you ever wanted to play a nasty loan shark, a venture capitalist, or an economic mastermind of any sort, goblins are the race for you.
It will be interesting to see if these possibilities do play out. These two races certainly seem to make the most sense as far as the various possibilities go, but in the end anything is possible. Even if these two races are not chosen for the expansion, roleplayers would do well to think about the spirit both races represent -- perhaps we can explore their themes in other ways as well.