I have another theory
Sora - Darkness
Because Sora is fighting the darkness and keeps it away from the world to save his friends, but the darkness is constantly trying to contact him. The unknown men in the cloaks, are appearing, and in FM fights him, and tells him he is not ready for this and that.
Riku - Present
Because in the intro, you see riku with the water wave behind him, reaching out his hand, but the water cant touch him, it touches Sora instead. Like the unknown man who just walks through Sora. Riku was trying to make Sora come to the right side. And in doing so, he has made a huge mistake because instead of dragging sora to the present, he has dragged himself towards sora, into the darkness. And as the original keyblade wielder, he is a good capture for the darkness.
BHK - Twilight
First because he wields the Oblivion and the Oathkeeper, equals Dark and Light. And because he is fighting a silverish enemy, maybe some form of twilight enemy.
Mickey - Light
Well, he is king Mickey lol. And he has entered the world of darkness undercover, trying to get Riku, the Original keyblade wielder out of it. He had to save sora from the darkness because if he would die, the system of keybladers would be broken, then one side would loose its wielder.
Point in all this, the different worlds keybladers have been mixed up. The present has lost its wielder to darkness, the darkness wielder is keeping himself in present as long as he can. Mickey is trying to correct this working undercover in the darkness, with the cloak and all, the bhk twilight kid is trying to save the worlds from collapsing from a side point of view from dark and light