No one was expecting Nintendo to announce Kingdom Hearts: Melody of Memory for Switch coming next month. In fact, many initially expected Kingdom Hearts 3 to be the end of the long-run saga between Sora, Kairi, and Riku. But thanks to the end of KH3 and its ReMind DLC pack, a new main series installment was teased, presumably Kingdom Hearts 4. As a result, it's clear that the franchise has many more stories to tell.
Kingdom Hearts 4 is reportedly in consideration, but because of the secret ending found in Kingdom Hearts 3, the ending of ReMind, and what's currently known about the upcoming Melody of Memory, fans are able to infer what the plot of the next main series installment will be. Likely, Melody of Memory will bridge the gap between the ending of ReMind and KH4, with Kairi being the centerpiece of this information.
Connecting KH3 and KH4
Despite being a character essential to the overall story of the Kingdom Hearts series, Kairi has only received a minor amount of cutscenes throughout the games to explain her side of it all. One of the biggest disappointments of Kingdom Hearts 3 was the Kairi didn't actually end up being a playable character, and though she became a trained Keyblade Wielder, she spent much of the game away from Sora's journey yet again and in the background. Thankfully, with the introduction of ReMind, the player was given the option to play as the leading lady, and she took up much more screen time.
Sora had spent all of ReMind trying to find the pieces of Kairi's fractured heart, and in the end, Sora mysteriously disappears. Perhaps he, once again, sacrificed himself to save Kairi's heart. Kingdom Hearts endings have always been vague. It looks like Sora's abuse of the Power of Waking caused him to fade, presumably bringing him to Shibuya, the setting of The World Ends With You, another Square Enix game that collided with Kingdom Hearts in Dream Drop Distance. It may not be connected, but in The World Ends With You, Shibuya is the setting of the Reaper's Game, where the dead compete to transcend and become Reapers.
From what's shown in the final Melody of Memory trailer, it looks like Kairi is working with Ansem the Wise to find Sora and bring him home, but it's no easy task. She's revisiting the worlds (via the rhythm game) looking for clues about Sora. From what's known about the Reaper's Game, it could be inferred that Kairi is attempting to bring Sora back from the dead, or simply bringing him back from the world he's currently in. The rhythm game will both recap the adventures of Sora throughout the years while both telling Kairi's side of the story and introducing brand-new cutscenes of the present-day issue. Through their research, this will likely bring about the introduction of Yozora.
Finding Yozora
Yozora was already introduced to the main series in Kingdom Hearts 3 through the fictional game Verum Rex. After the events of the ReMind ending where Riku comes in contact with the Final Fantasy friends in a search for Sora, Riku finds himself in a different district of Tokyo, but within Verum Rex. This possibly means that Riku will find Yozora looking for his friend. While he's doing this, Kairi is asleep while Ansem studies her heart for clues about Sora, but through trailers, it looks like some of Kairi's interaction with Ansem could be after she's awoken from this slumber.
While the bulk of Yozora's story will likely come in Kingdom Hearts 4, it looks like he holds some sort of key to saving Sora. "I will save you," Yozora says after seemingly defeating Sora in Shibuya, an interaction shown in the secret ending of ReMind. Finding him will likely be a big plot point of Riku and Kairi's journey in Melody of Memory. If it follows normal KH formats, this event would come at the very end of the game, if at all.
Kingdom Hearts: Melody of Memory will release for PS4, Switch, and Xbox One on November 11 in Japan and November 13 worldwide.