So, my theories have been scattered throughout the forums in different threads so I decided to start a threat with them. Yeah, it's long and a bunch of different crap. So go to the bottom for the Tl;dr version.
Sora, Vanitas, Ven
Going into the Mastery Exam, Yen Sid spoke of a so called "Keyhole of Sleep." Now from the first game, Keyholes are used to unlock or lock the heart of a world. In DDD, I'd imagine that the Keyhole of Sleep may possibly play a similar role, except with memories.
Sora has fragments of Ven, memories of Aqua, as well as whatever remains of Xion and Roxas inside of him. Ven's memories are stored deep within Sora's heart where even Namine can't look at them due to the deep amount of pain within them.
Now, let us assume that Vanitas is a part of the memories stored within Sora. Namine's tampering of the memories during the Chain of Memories story line could have slowly awakened him. Now in the Dream World, Sora will encounter Vanitas who shall continue to taunt him, in a way similar to Scar's Ghost did in KH2. Muttering lines of pain, referencing Kairi and Riku, simply toying with Sora who is being watched with Yen Sid.
Slowly and continually tortured by Vanitas, Sora begins to lose his cool. This could eventually jeopardize his title of Master by Yen Sid as Sora finally realizes he is careless and can't see that the line between light and dark is not as straight as he once thought. This could also be shown by the BBSv2 trailer, where it shows Sora disappearing, followed by Ven.
Memories of others. Finding the truth
The Keyholes of Sleep could also reference the memories of those that have been connected to Sora and possibly Riku. Each time one is found, it would be similar to how ReCoded was, showing little snippets of information that wasn't known before.
Something like this: In Traverse Town, Sora finds the Keyhole of Sleep for that world in the cafe in District One. After finding it, he is transported back to a memory of Xion and Roxas fighting the Darkside in Twilight Town, followed by Ice Cream.
For Riku, it could show portions of Xehanort's time in Radiant Garden. Him talking to Ienzo about persuading Ansem, Xehanort creating the Room of Repose or him releasing the heartless into the Realm of Light.
Not only would it be a creative way for clip scenes, it would also help tie everything back together prior to Kingdom Heart's 3. Brings TAV, SRK, RAXN and everyone together.
Anyways, Tl;DR
Vanitas is inside Sora's heart in Ven's tormented memories. He taunts Sora through the MoM exam, causing Sora to lose the title.
When finding the Keyholes of Sleep, Sora and Riku get memories of those connected to him.
I'll edit this if there are corrections needed or if I get a new theory.
Sora, Vanitas, Ven
Going into the Mastery Exam, Yen Sid spoke of a so called "Keyhole of Sleep." Now from the first game, Keyholes are used to unlock or lock the heart of a world. In DDD, I'd imagine that the Keyhole of Sleep may possibly play a similar role, except with memories.
Sora has fragments of Ven, memories of Aqua, as well as whatever remains of Xion and Roxas inside of him. Ven's memories are stored deep within Sora's heart where even Namine can't look at them due to the deep amount of pain within them.
Now, let us assume that Vanitas is a part of the memories stored within Sora. Namine's tampering of the memories during the Chain of Memories story line could have slowly awakened him. Now in the Dream World, Sora will encounter Vanitas who shall continue to taunt him, in a way similar to Scar's Ghost did in KH2. Muttering lines of pain, referencing Kairi and Riku, simply toying with Sora who is being watched with Yen Sid.
Slowly and continually tortured by Vanitas, Sora begins to lose his cool. This could eventually jeopardize his title of Master by Yen Sid as Sora finally realizes he is careless and can't see that the line between light and dark is not as straight as he once thought. This could also be shown by the BBSv2 trailer, where it shows Sora disappearing, followed by Ven.
Memories of others. Finding the truth
The Keyholes of Sleep could also reference the memories of those that have been connected to Sora and possibly Riku. Each time one is found, it would be similar to how ReCoded was, showing little snippets of information that wasn't known before.
Something like this: In Traverse Town, Sora finds the Keyhole of Sleep for that world in the cafe in District One. After finding it, he is transported back to a memory of Xion and Roxas fighting the Darkside in Twilight Town, followed by Ice Cream.
For Riku, it could show portions of Xehanort's time in Radiant Garden. Him talking to Ienzo about persuading Ansem, Xehanort creating the Room of Repose or him releasing the heartless into the Realm of Light.
Not only would it be a creative way for clip scenes, it would also help tie everything back together prior to Kingdom Heart's 3. Brings TAV, SRK, RAXN and everyone together.
Anyways, Tl;DR
Vanitas is inside Sora's heart in Ven's tormented memories. He taunts Sora through the MoM exam, causing Sora to lose the title.
When finding the Keyholes of Sleep, Sora and Riku get memories of those connected to him.
I'll edit this if there are corrections needed or if I get a new theory.