The end where she falls back into darkness after being dragged back to destiny island, the core of the devil swarm appears to be the heart of the world.
It gets destroyed after her skirmish and she slips back into the end of the world where she would eventually meet Ansem on the dark shore.
The Keyblade that was in the door appears to be one of the 7 shards of light from the fragmented x-blade at the end of BBS.
Mickey will hold onto this Keyblade for safe keeping, we see it at the end of KH1 and him using it as his main weapon for KH2 giving up the star seeker from BBS.
The scene of Yen Sid, Kairi Riku and Mickey, they're off to find the nobody of Axel - Lea and then use Riku's powers to tap into the darkness to search for Aqua.
The scene of Sora, Doland and Goofie coming into the picture - Yen Sid at first displays how he is upset with Sora, going on adventures that drastically take him off the sights he should be following and going on adventures recklessly.
He then goes onto compliment him however on his feats, saying he displays the true characteristics of a hero, with this they leave in wait for their next orders fro Yen Sid.
In the Gummie ship they're discussing how there is nothing for them to do they're bored and then Sora thinks back to what Yen Sid said about him being a true hero and go off to the Colloquium to visit Herc and Phil
now that Sora has been dubbed a true hero. The worlds thought to be disconnected and closed off appear to be open for Sora to navigate to so he and gang set off which is where KH3 will seem to begin as it's first world story.