It will likely be based on Japan’s Sky Stream and Towering Perfection. The logo is reminiscent of Rayquaza’s colors, who’s the mascot of Sky Stream.
The two Japanese sets will release on July 9th. Each set will feature 67 cards before secret rares. Towering Perfection will feature Duraludon V/VMAX and focus on Single Strike cards while Sky Stream will feature Rayquaza V/VMAX and focus on Rapid Strike cards.
The “Evolving” in the name could refer to Eevee Heroes or a reference to this set being an evolution of Roaring Skies. There’s going to be 50+ cards cut from Chilling Reign, so they may be thrown into Evolving Skies over the cards from Eevee Heroes. I also feel like a set featuring the Eeveelutions would focus solely on them rather than share the spotlight with another popular Pokemon like Rayquaza.
Update: There’s also the Eevee from Eevee Heroes that features Rayquaza in the artwork. Which gives credence to Evolving Skies including Eevee Heroes. If it does, perhaps those 50 cut cards would be skipped over again and saved for a future set.
At this time we just don’t know. If we don’t get Eevee Heroes in this set, it would either become part of our November set or our special 25th anniversary set. We’ll find out soon!
No further details are known about Evolving Skies at this time, but the set will release on August 27th. Last night we revealed a new “Battle Stadium Box” will release alongside the set.