Maleficent tells Sora and co. about the enemies with “hearts of pure light” and releasing the “seven keyholes of sleep”. Do these have some sort of connection with one another?
Nomura: You’ll see what she means when you play it, so please look forward to that. (Laughs.) The connections of the various mysteries and truths are what it builds up to.
I'd like to focus a bit on the statement I bolded withing the quote.
"Enemies with hearts of pure light"
Could it be the Dream Eaters? They are the only enemies in the Dream Realm after all.
So that whole Dream Eaters are the new Heartless speculation is becoming kind of literal now. Except they aren't creatures of darkness, they are creatures of light.
Also, if this were true, any released heart of pure light would turn into a Dream Eater.
Those with hearts of pure light are the 7 princesses of heart, children, and Ven.
Now, as we see, a person with a heart of pure light without their heart is left in an unconscious (and apparently unaging) state. Say their hearts did not find a host or got released from their host.
Their hearts will go somewhere.
I presume those hearts turn into Dream Eaters
This, however, did not happen to the hearts of the princesses, as their hearts were used to create the Keyblade of People's Hearts.
The Dream Eaters are probably Nightmares (for the sake that you are beating them).
So now if Sora and Riku defeat any, hearts of pure light will be released.
So they essentially will work the same way as Heartless.
This is their birth by sleep.
Well, that's my theory anyways.