Niantic have announced that the next Pokemon Go season, starting on 1st September and lasting to December, will be the Season of Light. Given how soon the event starts, we have very little detail (especially as there’s supposed to be an event this Saturday).
However, given the Lunatone and Solrock surprise mini event after GoFest, everyone is pretty sure that it will be sun and moon based.
Niantic are currently dropping mini teases for upcoming Pokemon to be featured this season on their social media channels, but as yet we have no idea on bonuses or possible events.