The lore of Kingdom Hearts has grown into an ever-expanding universe thanks to this all-time great RPG's basis in an already massive Disney universe. So Kingdom Hearts fan theories are a given, as is the case with every expansive universe, but this one offers a particularly broad canvas. What's more, Sora, the Darkness, and the growing list of Keyblades are set to return, sparking a whole new wave of fan theories about Kingdom Hearts 3.
In the third installment we're likely to learn how the Disney worlds fit together in this universe, who the mysterious King is, and what exactly is in the Master of Masters's box. There are so many unanswered questions left from a series that has more entries than Guardians of Light, there's only one thing to do: speculate.
At One Time, All The Disney Worlds Were Connected
There is a "Disney Theory" beyond the universe of Kingdom Hearts that seeks to prove all the Disney movies exist in the same universe. Well, one observant gamer points to a line from the Kingdom Hearts universe that essentially claims the Disney Theory is true, which extends to the universe of the game. "In Kingdom Hearts: Birth By Sleep, Kairi’s grandmother tells her a story of a time where all the worlds were connected in light. Darkness crept in when people began to fight over the light, and that is how the worlds became separated."
Pretty simple. Whether it's true or not, one can't argue the existence of said claim within Kingdom Hearts itself. The theory is pretty elaborate (if you care to read it), discussing light and dark balancing each other in the universe, but it all comes down to grandma's claim.
Sora Is The Mysterious King
Redditor /u/PresWelke's theory that Sora is the one King in the world of Kingdom Hearts is based on some pretty sound, if not straightforward, reasoning. "The symbol representing Kingdom Hearts itself is the crown on the Kingdom Key and Sora's necklace. Thus leading to the belief that Sora is the rightful king (hence why the symbol is in the shape of a crown)."
That alone may not be convincing, but this Redditor goes on to point out a couple more design elements in the series that give further proof. One is "the [Kingdom Hearts I.5] cover art" which "not only features Sora sitting on a throne of Keyblades, but he is wearing the very same crown that represents Kingdom Hearts." Then there's the χ-Blade, which is known to be comprised of two Kingdom Keys, one of which is indeed Sora's Keyblade.
Sora And Mickey's Keyblades Will Reform The χ-Blade
Redditor /u/MonkeyDLenny asks an important question: why has Sora been relegated to just another face in a crowd of Keyblade wielders when we were initially led to believe he was a Chosen One of sorts? Because he really is. This Redditor points out that "no two base Keyblades are the same" except for Sora and Mickey's:
Mickey found that key in the Realm of Darkness, while Sora was granted his in the Realm of Light. The two perfectly mirror each other with reversed colors. So why are they the same?
Because both keys are the result of the χ-Blade breaking.
The seven light shards and thirteen dark shards faded into their respective realms, each manifesting into a Keyblade and waiting for the proper wielder for each to appear. If KH3 is really the end of Sora's story, perhaps the χ-Blade will be reformed by the two keys merging, and it will be the key to Xehonarts end.
'Kingdom Hearts 3' Will Feature A New Type Of Enemy Like Nobodies And Heartless
This theory of discord user csoysauce comes via Redditor /u/Rum3637, and it's a strong one. Based on some solid evidentiary analysis of trailers for the third installment, this Redditor aims to pinpoint what exactly an integral new addition to the series might look like. The theory is long, but this paragraph wraps up the important stuff:
I believe the new enemy type will be a manifest of the characters and ideas in the specific world you are present in. A darkness taking over a familiar attribute in the world. Take a look at the concept art for Big Hero 6 and notice there is a Baymax taken over by darkness. Recall from Union χ the nightmare Chirithy from wielders that fell to darkness. In the toy story trailer there are mechs that attack you, most of which carry a purple/ reddish pink colorway. Within the most recent Toy Story trailer Young Xehanort also specifically says "One heart's shadows fill the emptiness of another. See how they bring them to life?" and then goes on to say "Like Heartless and Nobodies, they fit together."