I don't know if anyone has thought about this theory. I think that it was the time when the worlds were just one world. Then the 4 people that were shown were the first ones to find the keyblades. The first 3 characters wanted to use the keyblades for good and the 4th wanted to harness the power of the keyblade for his own purposes. This what causes the keyblade war. The 4th guy need more forces so he gave more people keyblades. The other 3 did the same. Then the war broke out. Then during the keyblade war, the world was splitting apart, and eventually that world was no more. Maybe the keyblades held the original world in tack. When the keyblades were being used for the war, it destroyed that world and created the worlds that you visit in the games. The only way to get the worlds back to being one is by returning all the keyblades from where they were.
Then Sora, Riku, Mickey, and Kairi are descendents of the good keyblade weilders. Sora, Riku, and Kairi could be descendents of the first 3 people shown in the clip. Xehanort or Zexion could have been desendents of the 4th person seen. I also think that birth by sleep means that they keyblade weilders will get there full powers and what their destinies are. Just a LONG theory. Don't get mad for posting another theory on the secret clip.
Then Sora, Riku, Mickey, and Kairi are descendents of the good keyblade weilders. Sora, Riku, and Kairi could be descendents of the first 3 people shown in the clip. Xehanort or Zexion could have been desendents of the 4th person seen. I also think that birth by sleep means that they keyblade weilders will get there full powers and what their destinies are. Just a LONG theory. Don't get mad for posting another theory on the secret clip.