The official Pokemon Account already hinted at the content. It's Kalos DLC, or at least Zygarde.

https://twitter.com/Pokemon_cojp/status/1762130406239830297?s=20

>いよいよ本日2月27日(火)はPokémon Day!
>『ポケモン スカーレット・バイオレット ゼロの秘宝』のイラストを公開!
>オーガポンとテラパゴスが見つめる向こうには何があるんだろう?
>Pokémon Presentsは本日23時から!
>お楽しみに!

>Today, February 27th (Tuesday) is finally Pokémon Day!
>Illustrations for “Pokemon Scarlet/Violet Zero’s Treasure” have been released!
>What lies beyond Ogapon and Terrapagos' gaze?
>Pokémon Presents starts today at 11pm!
>looking forward to!

The English tweet doesn't mention Ogerpon or Terapagos, and the image is clearly referencing Zygarde with the green hexagons in the shape of a tera star.

Johto and Unova both lost, Kalos won.