It's finally happening: the Sinnoh region is coming to Pokémon GO. Niantic showcased a short video on Twitter, featuring glimpses of the Sinnoh starters, along with what looks to be the legendary Pokémon, Giratina.
The video teases the starters from the Sinnoh region. We see Turtwig in his grassy habitat, Chimchar in a cave, and Piplup running around under the ice. At the end, what looks to be Giratina appears. No release date was given, but presumably it won't be too long before trainers are able to look for and catch a new wave of Pokémon.
The Pokémon GO blog states the game will be re-balancing certain battle mechanics, and that other things will be altered. According to the blog, weather will have a "reduced effect on the rate at which Pokémon appear," and that certain places, like parks, will feature a larger variety of Pokémon. Also, according to the blog, "as you explore a given area, you will notice that a greater variety of Pokémon species will appear over time and at different rates." This should prove to be a great thing for people who are seeing the same Pokémon in a given area. These changes (and others listed in the blog) are stated to be arriving "in the coming weeks."
The Sinnoh region was the main Pokémon world from 2006 to 2010, with the release of Diamond and Pearl. (An enhanced version titled Platinum was also released.) The region at this point is nostalgic for many. The Pokémon of Sinnoh were featured in Super Smash Bros. Brawl, along with the Spear Pillar stage. Palkia, Dialga, and Giratina were the primary legendary Pokémon of that era. Although not featured in the teaser, one can assume Palkia and Dialga will show up in Pokémon GO sometime after the Sinnoh region launches. Something interesting to consider is that Eevee was given two more forms in Generation IV: Leafeon and Glaceon. Presumably, you will be able to get these forms after Sinnoh launches.
Currently, Pokémon GO is featuring a "Psychic Spectacular" event, which runs until October 14th. If you want to stock up on Psychic types, now is the time. (According to the blog, you may be able to catch a Shiny Drowzee as well.) The next Community Day is scheduled for October 21st, where the Pokémon Beldum will be featured. This should be a decent holdover until the Sinnoh region arrives. Hopefully an official launch date will be announced soon.