I'm writing a KH3-style fanfic at the moment (link to my Fanfiction.net profile is in my sig) and I made a theory about what the keyblades are and how they choose people.

Basically, this links the the story that Kairi's grandmother told. When the whole world was getting covered in darkness, the remaining people dreamed for a way to defeat the heartless, and therefore the keyblades were born. Each keyblade wants different qualities from it's master, like Sora's values bravery and loyalty and all the good hero crap, and the Way to the Dawn values someone being sorry for their sins, and seeking redemption.