Niantic has announced that new Pokemon will be added to Pokemon Go in December, setting off a flurry of speculation among fans as to which ones will be added to the Pokedex.
The most likely possibility is the so-called Generation 2 Pokemon, which were originally added in 1999-2000 in the Pokemon Gold and Silver. They include such popular types as Cyndaquill, Porygon, and Tyranitar. This would both expand the range of possibilities for gym fights and duels, and give players who have completed their Pokedex much more to hunt for.
Another possibility is the Legendary Pokemon, a group of incredibly rare and powerful Pokemon such as Mewtwo and Latios. These would definitely affect the balance of power in the game and require careful implementation by Niantic.
Further announcements as to new additions to the game should be coming from Niantic soon.