Square Enix has not yet laid all the cards on the table for Kingdom Hearts 4, but fans have a pretty good idea of what to expect. The game sees Sora separated from his friends and thrust into a dimension that either contains or consists of a city called Quadratum. In his quest to right wrongs and eventually return home, Sora will team up with and fight a variety of new characters, even if the player might be able to recognize them. Kingdom Hearts 4 will be a look into what the future of mainline Kingdom Hearts games will be, and fans can't wait to hear more about it.
That's not to say that the old Kingdom Hearts has been completely done away with. There are also likely to be elements concerning known KH characters in Kingdom Hearts 4. The ending and secret ending of Kingdom Hearts: Melody of Memory elaborated on what Sora’s closest friends are doing back in their home reality. Riku has taken up the Power of Waking to follow Sora into Quadratum, and Kairi has returned to Keyblade training under Aqua. Donald and Goofy were shown in the Kingdom Hearts 4 reveal trailer to be looking for someone, though who is unclear. That leaves Disney's iconic mascot Mickey Mouse, and there is a good chance KH4 will make a point of giving him some screen time.
Mickey’s Story Could Run Parallel to Sora’s in Kingdom Hearts 4
In the Melody of Memory’s secret ending, Yen Sid sends King Mickey to Scala ad Caelum for the purpose of researching the ancient Keyblade wielders and anything they knew about Quadratum. Since Donald and Goofy are shown in what will presumably be a cutscene in the finished game, it makes sense that Mickey will be too. Whether he will be reading lore out of a book in an abandoned library or talking with actual inhabitants of the world is another matter, as the present state of Scala ad Caelum is unknown. Whatever the case, Mickey will probably unearth a few new details about the realm of fiction and possibly the Foretellers.
In addition, Mickey will probably be shown later on rejoining the main group of heroes. There is reason to believe that the old cast will discover a way to cross into Quadratum, and the game will end shortly before or after they arrive. This will have the dual purpose of bringing Sora, Donald, and Goofy back together, while also ensuring that Mickey can maintain some relevance in the upcoming events. It's difficult to speculate any further given that the end goal of Kingdom Hearts 4 has not been revealed, but Mickey and the rest have other ways of helping Sora across realities.
Mickey’s Bonds With Sora Can Grant Him Strength
Kingdom Hearts likes to remind players that Sora's friends are his power. The bonds between close friends and loved ones are shown to grant power beyond one's individual strength. Characters have been shown to defeat foes far stronger than them and even cross worlds in an instant. While all of that happens near the end of Kingdom Hearts 3, Mickey Mouse demonstrated a mysterious manner of interworld travel to Riku and Sora’s locations all the way back in Chain of Memories and KH2. In KH2, this temporarily replaced Sora with Mickey to spare players from death in certain major fights, and it would be interesting to see something like it again. It could be reframed as a Rage Form replacement where Sora summons the strength of the Keyblade wielders of light from Kingdom Hearts 3 Re Mind.
Alternatively, the Dimension Link mechanic from Birth by Sleep can make a comeback in KH4. The D-Link allowed Aqua, Terra, and Ventus the ability to swap their Command Deck for that of various Disney characters, as well as each other. These D-Links came with new finishing moves and abilities, and each could be upgraded into a new form. It would be great if these could come back as summon replacements in Kingdom Hearts 4, and would provide a reason for Sora to recall Mickey's acrobatic and light-focused abilities without the mouse being physically present. There are lots of ways for Kingdom Hearts to show Sora making use of his powerful bonds in gameplay, and even if the old cast don't end up doing much in KH4, their presence could still be felt.
Kingdom Hearts 4 is currently in development.