After nearly one year of Pokemon GO was released, eventually Niantic provide related information Legendary Pokemon in Pokemon GO.
According to the interviews of CEO Niantic, John Hanke who discuss the company's plans for Pokemon GO in 2017, Hanke comment gamers want to know about the next major update of Pokemon GO, the Legendary Pokemon.
Besides Legendary Pokemon, Niantic will also improve the experience of playing on the fight in the gym.

Companies recognize that they release the feature in an incomplete state so it does not reach the optimal level.
Niantic wants a fight gym more attractive and provide a greater incentive to accept a gift from a fight gym.
Hanke also said they would add the Legendary Pokémon like Mew, Mewtwo, Articuno, Zapdos and Moltres in 2017.
Niantic feel responsible for preventing the use of cheat and hope gamers did not do so can enjoy the game.