The December update to Pokemon Go is expected to bring 100 new Pokemon to the game. The update will bring in Legendary Pokemon along with PVP (player vs player) battles. The legendary Pokemon in this update will also include Mew and Mewtwo, finally.
No details on when in December the update will happen, but it will most likely be in the first week or two of December. The new Pokemon will most likely be “2nd Gen,” Pokemon, which is pretty obvious they would go this route.