Grand Unifying Guesses/IWBH Wiverse

Kingdom Hearts, Bleach, and The World Ends With You all take place within the same universe.
The cosmology fits together too well for it all to be a coincidence. Which shall from now on be known as The "It's a Wonderful Bleached Heart World" universe or IWBHWiverse for short.
 * As a correlation, every Disney and Final Fantasy story ever also takes place within said multiverse.
 * It should also be noted Minecraft takes place in this Universe.... with the original spawn point being Destiny Island.... and the entire IWBHWiverse being steadily built outwards from there.... until the Keyblade war (Or possibly an update) shattered the world creating the lanes between.
 * Makes more sense than you might think... Looking at Destiny Island it has most of the signs of an early Minecraft world, for example the cave with the door is where Steve spent the first night, followed by rudimentary shacks, boats and so on as he mined out the resources.... then blocked off the mineshaft wth the Door.

Alternatively, Lilo and Stitch: The Series is the Disney Keystone World.
In Canon alone, it's been firmly established that Recess, The Proud Family, Kim Possible, and American Dragon: Jake Long all coexist with our favorite scrappy human-alien pair.

Azeroth and Dreanor/Outland share a Multiverse with Kingdom Hearts.
Too many coincidences to pass it off as random chance. Draenor's floating landmasses and energy Vorteces are suspiciously akin to those in The End Of The World. The Burning Legion employs tactics similar to those of the Heartless, namely using beings made from negative emotion and fueled by Hunger to assimilate and destroy entire worlds that can be swayed by beings of strong will and power. It can be seen that the KH 'overworld' bears at least a passing similarity to that of the Twisting Nether, with its' flows of energy and worlds suspended within it. It probably wouldn't be far off to think that Illidari Demon Hunters or even Illidan himself trained Riku in the Demon Hunter arts to master his Darkness powers, and bestowed upon him the Blindfold to develop his Spectral sight. It's been shown that they are well-skilled with blades and can metamorphasize into their own Inner Demons(Ansem in Riku's case) in addition to blasts and barriers of dark energy.

Kingdom Hearts is the reality of the universe, just shown on a smaller scale; that makes every piece of fiction reality
Think about it. Loads of worlds, people travelling between them...who says they haven't passed by our own world? Or been to it?
 * Consequently, works of fiction are representations of events that actually happened on other worlds, and the creators are the lucky humans who've been showed the truth, and given the responsibility to tell the world about these other worlds, using fiction as a cover so as to not be seen as total loons. Of course, the message gets garbled in translation, and the "creators" don't give the world the hard facts about what happened.

== The KH 'verse is divided into different areas based on media; Disney Animated Canon and Final Fantasy are one of the few sets of universes that are enmeshed to the point that travel between the worlds by many different characters is possible. == See Kingdom Hearts for reference. There are individuals who serve as go-betweens for the main Organization XIII and the other sections of the universe, and rarely if ever go from one section to another. Possible representatives include:
 * Xion: Ino Yamanaka, a representative for Anime. Her soul/heart-throwing power, mostly. If it's not already jossed, it probably will be soon.
 * Xykon: Kyon. The original Anime representative. Went rogue and they dumped him into Web Comics, or possibly Tabletop Games.

The World Ends With You takes place in the same universe as Soultaker
In both of them, the afterlife is just vague menacing twenty-somethings in dark clothes trying to steal our souls, regulated by a mysterious unseen godlike figure, and people who can't be seen by shopkeepers running away from them.
 * But in The World Ends With You, the Reapers are trying to erase our souls, not steal them, and the players run towards Reapers more often then they run away from them. Also, the players can be seen by anyone as long as they're within the range of a modulator decal, and every store has one, so shopkeepers can see the players.

The game Mother 3 takes place in the same universe as the movie WALL-E
Consider that the Axiom in WALL-E is white. Given the themes of Mother 3, it could be inferred that

The Joker and Kefka are one and the same
This doesn't require much thought. But come on, look at what they have in common:
 * They are both clowns
 * They both have insane laughter
 * They are both homicidal maniacs
 * They are both omnicidal maniacs
 * They are both Chaotic Evil
 * They both outshine any other character ever whenever they're onscreen

They have to be the same person. It's the only way to prevent the universe from being destroyed when they meet. (Not that they would mind, but everyone else would.)

All we need is for Mark Hamill to be cast as Kefka for Dissidia Final Fantasy, and this will be vindicated.
 * Both characters are also regenerations of The Master.

The Oz books take place in the Kingdom Hearts multiverse.
In The Tin Woodman of Oz, the Tin Woodman is described as a Nobody.
 * But... the Tin Woodman has a heart.

Cecil and Rosa's Power of Love is more powerful than Black Mage's use of The Power of Love
See, when Black Mage siphons love out of the univarse and uses it as a beam of destruction, it takes the form of Hadoken, a ninth-level spell. Meanwhile, Cecil and Rosa use their love to power the Twincast spell Ultima, which Black Mage says belongs to the tenth level of magic, and is supposed to just be a theory. Therefore, Black Mage is weaker then their power of love.

Of course, Domon and Rain probably eclipse the both of them at once, but still.