Niantic CEO John Hanke is definitely teasing a Gen 3 release for Pokemon GO. While we don't have a specific date on when these Pokemon are coming, it seems like they could be soon.
"There is a lot of Pokémon that have not yet been launched. So I think the next secret weapon will be launch of the next Pokémon generation. I hope the players will soon see them."
Hanke also went on to talk about the revamped battle system, but it seems that's a ways off. According to him, that would be more in-line with a 2018 release.