B2W2 have increased the list of available abilities considerably. However, there are still 33 Pokémon whose abilities are currently unobtainable: Articuno, Zapdos, Moltres, Mewtwo, the Chikorita line, the Cyndaquil line, the Totodile line, Raikou, Entei, Suicune, Regirock, Regice, Registeel, Heatran, the Snivy line, the Tepig line, the Oshawott line, Gothita (but not its evolutions) and the Litwick line.
It is probably safe to assume that the non-legendary Pokémon will be unlocked via the Dream World by the end of this generation, but what about the legendaries? They have rarely been distributed via the Dream World. Why don't the Regis or Heatran have access to their abilities in B2W2? Why wasn't Mewtwo distributed with Unnerve? The only legendaries with obtainable hidden abilities are those that are available via the Dream Radar.
Personally, I doubt that Mewtwo will be distributed again, even though Movie 16 presents that possibility. I find it more likely that the shiny Genesect and Eevee will be the two movie downloads, representing both the movie and the short. I wonder if Mewtwo will be catchable in X and Y with a hidden ability that is actually useful.