Current rumors state that Pokemon Go players can expect to see the next generation of Pokemon coming in May 2017; this has not been confirmed by Niantic but several sources are all reporting this month as the release date.
The rumors that state this have also provided other release times for other features; Pokemon Trading will be released this October and Pokemon Trainer Battles is expected for a December or January release. If these other dates are correct then we can expect to see the Johto region Pokemon coming in May 2017.
It’s expected that the next generation will also contain some region locked Pokemon and players shouldn’t be surprised if Niantic keep Unown out of the initial release; the Pokemon is not quite looked as being a legendary but it is very mysterious and there are 28 different versions of it. Niantic will most likely release them for different events so they won’t be released with the rest of the generation. This is just speculation however, Niantic may decide to release them with the rest of the generation or they could release them to tease the release of the Legendary Beasts from Generation 2.