The current speculation is that since Goblins have a reasonable explanation for joining the Horde (*), Worgens would be the playable race for the Alliance. This is heavily discussed and most generally the reasoning for this would be that Worgen are not specifically evil (despite what other sources may say). Another reason could be that this is a "special" breed of Worgen, but that's kinda lame.

The other theory is that, with the new faction change service, Goblin and Worgen will be given their own faction, a neutral faction, along with a possible two other undiscovered races. This gets my vote, because that'd be interesting.

A third theory would be that neither new race is given a specific faction right off the bat and instead a quest chain in their respective starting zone allows them to choose which side they want to be on. This is widely regarded as one of the most interesting theories but I don't think it holds much ground personally.


* - According to the lore, a break within the goblin faction led to a certain trade prince taking a bunch of goblins away from the neutral side and striking out on his own. A likely reason for these goblins to join the horde would be due to the recent Kul'Tiras activity which is supposed to be an available area in the new expansion.

/hugenerd

There is a lot of speculation on this. It was all started when someone (MMO-Champion, maybe) datamined some new Halloween masks for both Goblin and Worgens. Halloween masks are typically reserved for playable races.