Update, October 16, 2018 (12:15PM EST): Niantic confirmed that Pokémon from the Sinnoh region will arrive in Pokémon Go this week. The mobile developer also confirmed that the new Pokémon will arrive in waves and that they will appear in the wild, in raids, and in hatched Eggs.
Original article: We caught wind of this back in April, but mobile developer Niantic finally confirmed that fourth-generation Pokémon from the Sinnoh region will soon be available in Pokémon Go. This means that you will soon be able to capture Pokémon like Turtwig, Chimchar, and Piplup (my personal favorite).
Those specific three are assumptions, however. Niantic did not specifically state which fourth-generation Pokémon or how many of them, will be available in-game. We also do not know which Pokémon will be specific to which region or when they will arrive.
— Pokémon GO (@PokemonGoApp) October 9, 2018
Alongside the Sinnoh-region Pokémon will be some changes to Pokémon Go that are supposedly based on player feedback. Those changes include weather having a reduced effect on the rate at which Pokémon appear, a greater variety of Pokémon that show up over time and at different rates in specific areas, and more varied Pokémon in areas like parks and nature reserves.
Changes also include adjusted CP and HP values, retroactively-balanced Defense and Stamina values, and retroactively-reduced Defense values for most Pokémon. Some people have not taken kindly to the announced changes, particularly those who use Blissey and similar Pokémon.
The Sinnoh-region Pokémon, along with the in-game changes, should be available sometime in the near future as an update. In the meantime, you can download Pokémon Go at the link below to see what the hype is all about.