My theory for Kingdom Hearts 3D is that after sora and riku are informed of what happened in coded they search for the one who connects all these people togethor, Xehanort. Think of the Kingdom Hearts Universe as a large Bubble and within this bubble is the realm of light, realm of darkness, and the realm of inbetween. Beyond this bubble is the realm of nothingness, and beyond that is another bubble/universe/dimension. Sora and Riku find this dimension in hopes that Xehanort may have hidden there. Once both sora and riku enter this dimension they find themselves with their KH1 appearences and a whole alternate reality universe around them to explore. thats where the new worlds, the twilight thorn in traverse town, and the falling soras come into play.