First up, Goblins. As mentioned time and time again, on Two Wolf Moon and countless other sources debating this topic, Goblins are vastly a neutral race. They are driven by greed, so it's bad business skills to deny potential customers. Thusly, many are quite certain about the Goblins being unable to take sides. I can definitely see why people would think that and, for the most part, I agree with many of their points. However, if a splinter faction of goblins were to officially join the Horde, I'd imagine a similar scenario would take place...
These goblins and their trade prince would probably be quite the black sheep within Goblin society for some form or another. All cultures have their exceptions (The Stormwind Humans have the Defias, the Tauren have the Grimtotem, the Ethereals have the Etherium, and so on...) so I feel that saying that all Goblins are the exact same would probably be doing them a great injustice. Take a look at the Venture Company--Now there's a goblin organization that is friends with neither Alliance or Horde. If there are untouched lands with promises of natural resources or forgotten treasures, they will be there tearing things up and killing/enslaving all who get in their way. So if the Goblins can have an organization that is the equivalent of Hoggish Greedly, that is hostile to both sides, then I find it possible for the existence of a Horde-friendly Goblin group. Not to mention that the goblins, in general, have been on pretty good terms with them despite their neutrality.
But as I mentioned before in one of my previous ramblings, they'd have to be in some serious hot water first. Whatever the cause may be (Their "base" under siege by Naga, economic desperation, and so on...), these hypothetical Goblin allies would need to be in so much trouble that they would have little option but to crawl up to Thrall's boots and kiss them.
On the other hand, why would Thrall take them in? First up, Thrall is not a stranger to helping, at the time, would-be allies out of a sticky situation (IE: Tauren and Darkspear Trolls). Even though these new goblins would initially be more trouble than they're worth, having a technologically driven race that would be honor-bound to develop weapons and vehicles only for the Horde and not sell them to outside parties is a pretty tempting deal...
LONG STORY SHORT: Would Goblins make the best probable addition to the Horde? In my opinion, there are better ones, but if this expansion is indeed involving the Maelstrom, then it gives the Horde yet another reason to go out towards the Rift since Kezan would be under attack. Not to mention that whole Darkspear Islands incident a decade or so back... I'm pretty sure the Trolls are mighty keen on exacting revenge on the powers that drove them out of their home.
On a technical note, as previously mentioned, the addition of Goblins to the Horde would give them a shorter, possibly healer-less, race that is technologically driven--Just like the Alliance with the Gnomes. And Blizzard is all about the whole equality schtick.
Now for the Worgen... I will say, off the bat, that this race is a hard one to pin down in comparison to the possible addition of the Goblin race. My previous entries regarding playable Worgen are, of course, purely speculation combined with a generous helping of not-so-silent wishing that this rumor is indeed true. This much I realize, however I still do honestly believe that if the Worgen are indeed slated to join the Alliance, it's not impossible--Just difficult.
First off, if the worgen are to be a playable race in this coming expansion, then I sincerely doubt it'll be the "space wolves from another dimension" variety that was brought to Azeroth via the Scythe of Elune. They are savage beasts who'd much rather kill you than befriend you, so unless something tremendous happens to magically change all this, I don't see them changing their stripes anytime soon.
No, the type of Worgen I'm banking on are the cursed variety... Namely those seen in Pyrewood Village and Grizzly Hills. Though I will say that I see a problem with this theory, right from the get-go. Pyrewood's residents only turn into werewolves at night and the Wolfcult variety appear to have the ability to shift at will. If Gilneas is indeed added in the next installment of WoW, and its citizens turned into wolfish monsters, I would imagine they would have to be fully turned and completely feral. So this is where good ol' Alphus comes in, providing he is indeed a known figure within the game, and helps change that with his Alliance-funded venture to help the Gilnean's regain their sense of self. But even then... Would they join?
Theoretically being turned into worgen-like creatures aside, the Alliance and Gilneas hasn't always seen eye to eye on things... Genn Greymane and his people are notoriously stubborn and prideful. Even though their once-powerful nation would be in shambles, would they admit that they were wrong and that they need help? Would King Fighty McChin actually let a bestial race in with the Alliance that has had a rather bad history with them (IE: Essentially Greymane saying "Screw you guys, we don't need no stinkin' Alliance to help make us awesome. We're already there!")?
I think it'd definitely make a very interesting addition, at least story-wise, if that were to happen. Heck, I'd think Varian would find Genn's possible new form to be appropriate to his attitude. Besides, if the Allies didn't pick them up, they'd probably (begrudgingly) join the Horde and add to their numbers. That's a big no-no to them right there...
LONG STORY SHORT: Given the right explaination, I'd think the possibility of Gilnean "worgen" is a strong one. It would give the Alliance a wild card race, much like the Horde's Forsaken, and help stir things up a bit. However, I cannot see much reason for them, at the time, to join the fight aside from just being desperate for help. That is unless there has been reports of the Scythe somehow slipping out of the Wolfcult's grasp (that is where it is presumed to be currently) and ended up somewhere in the Maelstrom. I honestly do not know... Regardless, I'm sure all our theories will be cleared up come Blizzcon.