Blizzard has decided to remove the one faction per PVP server restriction to accomodate their planned paid faction change service. This is most likely because the alternative, changing the factions of all the player's characters on that server in a single transaction, would lead to cries for the same bulk discount to be available for other account transactions. However, what if there were a second reason why the change was needed?
Blizzard made a clumsy attempt to cover up the Worgen and Goblin Halloween masks by adding a bunch of miscellaneous masks in the next PTR patch. Still, most people believe that the expansion will see Goblins added to the Horde and Worgen added to the Alliance. Could they be neutral races instead?
If you were ever going to do a neutral race, Goblins would seem to be the best choice. They will work for the highest bidder, and they already make most of their towns neutral, so it wouldn't be a stretch to imagine subfactions of Goblins in the employ of either of the two sides. If Blizzard REALLY wanted to shake things up, though, they could allow Goblin characters to SWITCH sides.
The Goblin mercenary agency could have a policy of wiping the short term memories of its troops before hiring them out, removing faction-specific languages and reputations. The player could be required to foot the bill when they request reassignment, and the switch could have a cooldown that represents exposure to the radiation of the mind-wipe device. This would prevent the worst of the trash-talking abuse, while giving the new race a unique feel (and access to all the faction mounts in the game, a potentially fun perk).
As to the Worgen, the speculation centers around the formerly human kingdom of Gilneas, one of two locations in Azeroth (the other being the Grizzly Hills) where Arugal appears to have inflicted a Worgen curse upon people. (There are a separate group of Worgen in Ashenvale and Duskwood who have been summoned from somewhere, and who may or may not still possess non-Worgen forms.) The Alliance questline in the Grizzly Hills suggests that characters - even the non-human ones - can be turned into Worgen. What if Worgen were a CLASS option representing characters who have been cursed, rather than a player race?
Obviously, Blizzard might be hesitant to add a new class or classes after dealing with the balance issues of adding Death Knights this expansion. Then again, have we ever seen Worgen using weapons, running around with pets, etc? As far as I can remember, basically all Worgen mobs are either unarmed melee combattants or shadow mages. What if you added a melee Worgen class that consists of a Warrior whose Berserker stance is replaced by Worgen form and a caster Worgen class that consists of a Priest whose Shadowform is replaced with Worgen form?
The Worgen player would be indistinguishable from another member of their race/class until they suddenly went berserk (presumably with some benefit in place of the normal racial perks). This would have minimal effects on class balance, since you wouldn't really be adding new classes. Perhaps players could even be given a way to infect their existing characters if they were so inclined.
Alright, realistically, I don't think anyone is expecting anything so far out of the box. Far more likely is that Goblins and Worgen will be single-faction like any other race, and the change to PVP server restrictions was simply to make it easier for Blizzard to justify charging per character for faction changes. The only thing we might see if Blizzard is feeling ambitious would be for the new races to start somewhere around level 40, with their starter content available for alts of the other existing races to help pad out the relatively sparse 40-58 content range.
Still, I'd be far more interested in something like this that really offers a different experience instead of yet another race that plays just like all the others.