An easier possibility is, they're simply expanding the mask hunt for Hallow's End, way before the next expansion. Rather than collecting masks just for the playable races, we'll have an achievement for running around getting non-aligned ones too. There was enough protest over the lack of newness in Midsummer's ... perhaps they want to keep the seasonal events fresh for those who have done them before?

As for the expansion, goblins are neutral, right down to the bone. They worked for the Horde at one time but are a truly neutral institution in WoW, as much so as the Cenarions or Argent Dawn. It would be strange and kind of lame (and disappointing) for them to suddenly take sides. Blizzard can make it happen, obviously, all they need to do is write it down and it's reality, but I think it'd be very hard to revise the image of goblins to fit the Horde, so I think they'll be more creative than that and come up with something new for the expansion.