Niantic have announced the next possible Pokémon GO event. If enough people participate in their Earth Day clean up from April 13th to April 28th, then there will be various bonuses introduced in Pokémon GO
If 2000 attend, there will be an increased appearance of Ground-type Pokémon.
If 5,000 attend, then there will be an increased appearance of Ground-type Pokémon and shiny Diglett will become available
If 7,000 attend, then will be an increased appearance of Ground-type Pokémon, Shiny Diglett, 2* Catch Stardust & Candy for Event spawns and Groudon will return to raids.
Niantic have announced various ways to sign up to participate on their
official site and players with small groups or on their own can tag themselves in on Twitter tagging @NianticLabs in #AugmentedReality. We'll provide further details as they come