Er...Final Fantasy VII and X! Tidus and Aeris they died!!!! And Aeris died on the first disc! And in Star Ocean, they killed off Fayt's dad to make the moment more dramatic so...yeah.
And some people said that Disney wont let them kill off a character but HELLO! Mufasa from Lion King, Kerchek from Tarzan, Kokoum from Pocahontas, Bambi's mom died from er... Bambi.
And it's not much of a theory but more of a hunch.
Like All the unknowns are apart of Riku or something and as Sora kills them of, Rku will slowly get weaker until he croaks (Eventhough I don't want him to)
In ASAS one of the quotes said "So you're the source of all the heartless?" and maybe that's Riku and Sora will have to kill Riku to stop the heartless.
Yeah I know there are a lot of reasons that disprove this but hey I said it was a hunch not a theory.