The Pokémon come from the Johto region in the Pokémon Silver and Pokémon Gold games, and will include both new Pokémon and new evolutions. Gender-specific variations are also being implemented.
Some Johto region Pokémon have been previously added to the game through various holiday updates and promotions, but this will be the biggest update to the Pokémon Go since it was first released last summer.
New berries are being added to the game, along with new items that are required for some of the Johto Pokémon evolutions. Both will be available from PokéStops. The berries will include the Nanab Berry, which will slow a Pokémon's movements to make it easier to catch, and the Pinap Berry, which doubles the amount of Candy received if the next capture attempt is successful.
Pokémon Go is also gaining new encounter gameplay, and Niantic says Pokémon encountered in the wild may "react in new ways" when you're attempting to catch them. A new item carousel will also allow users to select berries and Poké Balls without needing to open the backpack.
The final new feature coming this week is an expanded wardrobe and new outfit options for a player's avatar. A new selection of hats, shirts, pants, and other items are being added to the game.
Excitement for Pokémon Go has died down in the months since its release, but the game continues to have a loyal fan base and Niantic has been committed to making regular feature updates to keep players engaged.
Pokémon Go can be downloaded from the App Store at no cost. [Direct Link]
Update 4/16: The Pokémon Go update that adds 80 new Pokémon is now live.