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Thought that this could be a hint because the temple pillars were a hint about Dialga and Palkia, but I did some digging myself (haven't played the DS/3DS games), and this just looks like the Dazzling Caves and Stargleam Cavern from Brilliant Diamond and Shiny Pearl. So probably not actually a hint. Maybe the next weird background we get will be the real hint.
>The enchantingly crystalline terrain found in these hideaways is home to an eclectic mix of Psychic types, Ghost types, and Fairy types. Despite this terrain being somewhat rare, there’s a long list of Pokémon that will only appear here, including Kadabra, Gastly, Hypno, Togepi, Wobbuffet, Girafarig, Snubbull, Ralts, Kirlia, Duskull, and Dusclops. You can also find Mawile in Pokémon Brilliant Diamond or Misdreavus and Sableye in Pokémon Shining Pearl. Keep in mind that this is the only terrain where you can catch Ditto, a Pokémon that’s crucial for Trainers hoping to discover rare Pokémon Eggs.
The only Pokemon listed here that we don't have already are Wobbuffet, Girafarig, Snubbull, and Sableye.
