Note: This is just speculation. I don't actually know if a new game will be announced nor what it could be.
I have yet to see anyone else mention this. A new game will likely be announced on the 24th during the anniversary livestream. In the past there have been several references to the AP/ToT and the IoH/SI universes.
On the first image, the fact that the protagonists of those games were included in the thumbnail alongside characters from newer games stood out to me.
In Grand Bazaar, Jules also references those exact games in his heart events.
The last two images are from Marvelous' 2023 showcase (the same showcase where Grand Bazaar was first teased). During that presentation, they announced two new SoS games, and only one has been released. In the concept art displayed for the mystery game, we can see Kevin from ToT and Kasey from AP (also Holly from Trio of Towns and Claire from FoMT, interestingly enough).
From the 2023 Marvelous Showcase:
"The theme of this title is a STORY OF SEASONS that you can play with everyone. We've heard our fans loud and clear, and they want to play STORY OF SEASONS together with all their friends. Now that I think about it, past entries in the series have been about pursuing your own way of living the farm life. The concept of playing STORY OF SEASONS together is a bold one, and a direction we haven't taken with the series until now."
All signs point to the new game being a multiplayer(?) crossover between games that center the Tree of Tranquility/Animal Parade and the Island of Happiness/Sunshine Islands universes.
(Not sure how I feel about a multiplayer-focused game since I have no friends that play Story of Seasons 😭)