In the Canalave Library there is a Book that was not in the original games for the DS, called the "The Sea's Legend", that the game describes as being "recently discovered" and that states the mysterious following text:

Spoiler:"Once upon a time in the East Sea, there was a Pokémon known as the prince. A brave human asked Pokémon living in the sea to let them see the prince. Mantyke, Buizel, and a Quilfish with huge spikes acknowledged the human's bravery and joined them. Together, they set off in a boat over the sunset-streaked sea, sailing through the ocean gate stretched over the waves. News of this reached the ears of the prince, who went to meet the brave little party at the Seaside Hollow."

It's possible that this text was put into Brilliant Diamond and Shining Pearl Spoiler:to tease the then upcoming game Pokémon Legends: Arceus via giving the player a hint of a storyline or event that may happen in the latter game's plot. Furthering the suspicious nature of the text, Game Freak, when sending out review copies of the games, forbid any reviewer from talking about any of the text in the library.

The time on the Pokétch from the DS in the first trailer says 9:28, a reference to the release date of the original Diamond & Pearl games.

Prior to the official announcement of Yoshi's Crafted World's title, a fake leak was posted on 4chan listing off future Nintendo games, containing the title of Yoshi's Crafted World. This leak was humorous in nature, contrasting legitimate sounding titles such as Animal Crossing: Town Founder and Paper Mario: Clay Catastrophe with obvious jokes such as Metroid: Equestrian Races and Rhythm Heaven and Hell. This led to a short-lived theory that the leak was indeed real and slid in the joke names for humorous effect. Over time, reveals of installments in other franchises featured in the leak that didn't line up would show that Crafted World was simply a lucky guess. However, some other articles from the leak, such as a multi-part DLC pass for Mario Kart 8 Deluxe, Pokémon Diamond and Pearl remakes, a Tokyo 2020 Mario & Sonic Olympics game (albeit without Pac-Man), a Switch port of Overwatch, and the at-the-time-believed-to-be-cancelled Pikmin 4 would come into fruition later by coincidence.