According to the Secret Ansem reports the Emblem Heartless are created using a machine, but there are far too many for all of them to be Xehanort's test subjects. My theory is hearts consumed by Emblem Heartless become Emblem Heartless themselves.
A lot of people seem to think Wreck-It Ralph's been ruled out as a possible world because of the title character being confirmed as a summon, but don't forget Genie was a summon in both the first game and KH2.
Master Xehanort's planning a huge fight between The 13 Seekers of Darkness and the 7 Guardians of Light to summon the X-blade, but if the whole Organization are still around by the end of the game the last level would be one huge boss rush (at least 13, to say nothing of whatever alternate forms...
Most people think Vanitas is a member of the Organization, and him wearing a black coat supports this, but what if this is just to mislead us? Ansem the Wise wears a black coat but there's nothing to suggest he's a vessel.
This was a theory I came up with recently, the new Marluxia-themed nobody appears in the Tangled world, plus most of the local heartless are plant based. This makes me think Marluxia's somebody might make an appearance, maybe working with Mother Gothel.
Everybody has FF characters they like and hate, Squall's always been a personal favourite of mine. My most hated FF character has got to be Gerrith Barrington from Tactics, in case anybody's wondering why, he habitually rapes a 13-year old girl and it's implied that he's impregnated her and he...
The 13 darknesses have six unidentified members, I was wondering if anybody has any theories on who they are. I personally think possibilities include Master Xehanort's nobody, Braig and/or Isa's Heartless and at least one Disney villain.