Okay i have been doing research about some footage in that kh2 secret trailer video, the one thing that grabs most of my attention is the word "KEYBLADE WAR"
well going by facts and fictions i have read i can present to you the following in formation.
For starters for there to be a keyblade war, there has to be more than one keyblade weilder. But going by information in the first game, there is only one keyblade wielder, but now we discover there is quite a few.
So its simple there will be many users of the keyblade in kh3, like many characters will be using keyblades. eg squall, cloud etc.
Other facts to note:
In the video clip you see those three knights, two guys, one girl. And most of you have noticed that one is roxas, even though its not him. What they actual are is the previous owners of the keyblades which now are Sora, Riku and Kiari. The only evidence that supports this is you see King Mickey with the star seeker keyblade, the one that wizard gave him and going by nfo mentioned in the game he was the on who taught him everything he knows, so this bit is before he met sora, so therefore all of this events that happened in this video were of the past. Hence it supports my theory.