Then again, after the credits are done rolling in KH2, Kairi runs out onto the beach with a letter in a bottle. It bares King Mickey's emblem on it. The three read the parchment with worried expressions, the camera never exposes the paper to show us what is on it. This is indication enough that Sora is being called off on another mission
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KINGDOM HEARTS 3 STORY COULD BE:
Years before Sora's adventure, there was a great war. In a place between worlds, there were those known as "Chasers", those who had manifested great power and chanelled it through weapons they experly crafted known as the Keyblades. There were four Chasers.
The Chasers all had to power to create, save, destroy and lock world's hearts. Their keyblade gave them enough power to unlock the energy of a worlds physical heart and be able to move from world to world.
The Chasers began to follow their own ambitions. In the place between worlds, where all of their keyblades laid to rest, the four seperated. Two followed the path of light, bringing life to the worlds, following the paths of Light and Twilight. The other two, they grew unhappy and decided that those who did not respect them should perish. They followed the path of Darkness. They began to bring destruction to worlds, destroying them.
One of the two Dark Chasers realized the faults in his ways and decided to back out. Much as Riku did, this Chaser follows the path of Dawn back to the Crossroads where he is forgiven for his ways. The final Chaser, the one they called Xehanort, would not give up, would not give in. The war began, it is unknown for how long this terrible time lasted. It has been recorded though and the Chasers are much like gods.
The war was either between great armies or just between the four Chasers, either way, the Dark Chaser, the Dark Keyblade Master known as Xehanort fought against the others in a great Keyblade war.
Xehanort was defeated, and with that, he lost something dear: his memory. Cast away he was born again by sleep. Waking up in the world of Radiant Garden, almost dead, a great Saint known as Ansem found him and offered his assistance. He promised to help him and when he realized the man's memory was gone, he offered to help restore it.
The Chasers each had the ability to spread their power to a disciple or their spirits lived on through a bloodline or chosen ones. Passing their power on to make sure to Order of the Worlds was kept safe.
One Keyblade went to the King of all the Worlds: Mickey.
When Xehanort began to believe he was in fact Ansem, his fascination with the Keyblades grew. Unknowning even to himself who he was.
When the Heartless were born and began to create chaos, Riku was chosen to weild the next Keyblade, to save the people from destruction while Mickey was in the World of Darkness. But, Riku's heart was tainted by his willingness to let darkness overcome him and the Keyblade was passed on to Sora.
After being selected first and later overcome by Xehanort, Xehanort was able to harvest his power to reclaim his Dark Keyblade and thus, Riku was able to have his own.
The final Keyblade was unnecessary until later, when the goodness and innocence of Kairi was able to bring hope to the other Keyblade Weilders and she too was chosen.
Roxas only had ones because he is an element of Sora.
After their adventures had ended, Mickey sends a troubling letter to Sora, Kairi and Riku. A letter that reveals Xehanort's next plot. That he has perhaps remembered his true self. A plot to destroy the other Chasers, a plot to be the one and only Keyblade Master.
A great beast is sent to the Crossroads between worlds but is slain by one of the Chasers. The others appear from their selected paths and reclaim their Keyblades, as though they haven't been touched in a long time. They look to the road of Darkness and the shady figure of Xehanort moves forward. Through his time within Riku, he is essentially "reborn" and while Riku tries to remove him, he is never gone. With enough power he returns to the Crossroads and a great war is about to begin once again. One that will decide the fate of the Chasers and all of those who hold a Keyblade...
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CHARACTERS:

Terr:
Terr is a character seen in the secret movie as the oldest of the three soldiers. He has dark brown hair with styling reminiscent of Xehanort, and blue eyes that appear to also glow with a goldish-amber hue. He begins the battle with the Unknown Elder Mage (unofficial acronym: UEM), and incidentially suffers the least damage from the encounter. Working together with the Aqua and Ven, he goes up against the UEM, and toward the end of the epic battle, removes his helmet to gaze up at Kingdom Hearts above him.
Terr was first seen in "Sunset Horizons", and was the first new character seen. He was standing triumphantly on top of a monstrous carcass resembling a Nobody, and picked up Sora's Kingdom Key from the dust.

Aqua:
Aqua is another new character, appearing to be a bit younger than Terr but older than Ven. She has vivid blue hair and blue eyes, and appears to be concentrated in magic more than physical attack skills. She casts Reflega quite a few times during the course of the battle against the UEM and DS. She appears to have a close connection to Ven, as seen when she and him lock eyes as he battles with the UEM and her reaction when his frozen body is flung to the ground beside her.
Aqua was also seen in Sunset Horizons--she picked up Mickey's Keyblade.

Ven:
Ven appears to be the youngest of the three warriors, but this doesn't keep him from showing his battle prowess. He bears a striking resemblance to the character Roxas, with identical hairstyle, facial features, and the same blonde hair and blue eyes, but this resemblance is still unexplained. It is unknown as to whether he IS Roxas or some form of him. The Blonde Haired Soldier concentrates on physical attacks most often, and during his faceoff with the UEM, he suffers the wrath of a Blizzaga spell throughout his entire body. Just as the battle turns in the UEM's favor, he locks eyes with the female soldier far below, who watches the battle in horror. After part of his helmet shatters and the UEM lifts him clear off the ground by the neck, he is frozen solid and tossed off the cliff, his Keyblade and parts of his armor shattering as he falls. The female soldier rushes to his side and lifts his frost-ridden body into her lap, shouting at him to see if he is all right, but all he can do is move his eyes to show her that he is still alive.
Ven was also seen in Sunset Horizons--he picked up Riku's Keyblade, Way to Dawn.

Unknown Elder Mage:
The Unknown Elder Mage (unofficial acronym: UEM) is the mysterious enemy who approached the three knights in Sunset Horizons. A pivotal aspect about his character is the parallel to Xehanort's Heartless--he appears to be wearing the exact clothing that KH1's main villain wore. He appears to be skilled in both Keyblade wielding and magical attacks. He can also manipulate the environment, as seen when he creates cliffs beneath his feet. He can also manipulate the massive number of Keyblades littering the battlefield to fly in formation. The UEM, despite his appearance, has great physical strength and easily overtakes all three of the soldiers in some fashion. His most critical hit came when he froze Ven from head-to-toe and tossed him off of a cliff to Aqua below.
UEM also creates a dark ball of energy and launches it into the sky, summoning what appears to be Kingdom Hearts from the clouds.

Dark Soldier:
The Dark Soldier bears resemblance to both a Nobody and Riku's Dark Form. It spawns directly from the UEM, appearing as a mirror-image of him and then solidifying into this odd dark-being hybrid. It appears to specialize in magic spells, and mostly stays by the UEM's side to assist in the battle. It has the ability to "ride" the waves of Keyblades that the UEM manipulates.
The Dark Soldier was not seen in Sunset Horizons, as he did not appear until the UEM was in clear view.

King Mickey:
Mickey appears from a hill, quite a ways from the battle. The mouse walks to the edge of the cliff, and watches as a heart-shapped moon appears from the sky. He is dawning a new black outfit and is wielding the Star Seeker Keyblade. His face goes serious as the trailer ends.
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KH3 REALEASE DATE:
As for a release date. Well, let's look at Square's track record. We saw the secret KH2 trailer after the first game in 2002. Then Square officially announced the game a year later. So that's about 2003, then they delayed it several times until they finally pinned down March 2006 for the release. That's FOUR years after the first game, people. And now, on PS3, they will have to start from the ground up getting used to developing for a new, more powerful system. Square will have to create a brand new graphics engine and battle system. That should take quite a bit of time, but don't complain, quality takes time, the more time taken, the better KH3 will be.
KH2 was delayed because game director Tetsuya Nomura was working on multiple projects such as KH: CoM and FFVII Advent Children. He has said that he will be working on another, unrelated game before Kingdom Hearts III. Check out my calculations:
Developing new graphics engine + new battle sytem + switching platforms + delays + possible delays of PS3 + other stuff = 4+ years.
I'm going to estimate we will see some previews of Kingdom Hearts III in 2008, then we'll see a release in 2011 at the earliest. WHOA! That's far off. But think, it's only 4 years. Let's all wish for sooner, and put some coffee on while at it.