These are my theories about KH3D based on everything I've seen/read about the game. I'm pretty much throwing it out there, so some things may contradict others. Consider it things to think about, and "what if" scenarios. Also, some of this info may have already been confirmed/denied. If so, just let me know. Again, I'm just going off of what I know in no specific order. Also, some of it may be obvious.
Cross-Out=MIGHT still be possible but unlikely.
Underline=Still possible in my opinion.
Bold=New and/or changed theory
Underline/bold=Confirmed in some manner
-Sora and Riku are taking the mark of mastery exam in an induced sleep. This also constitutes saving the world again.
-Organization members' "people" death are explained/how they became nobodies in this game
-I think Terranort took them out.
-Lea's grown up, and his organization robes are just his attire, as the organization didn't exist. It's just his normal clothes. (HA. I WAS RIGHT, mostly.)
-Getting out of the dream is part of the exam/completing it.
-Some worlds may be caught inside a dream world and can only be "released" if Sora/Riku seal the keyholes in their sleep.
-This may be part of the exam (It was already said, something along the lines of "Some worlds are trapped"...or something.)
-Neku and friends may be stuck inside the dream also, not just figments of Sora's imagination
-Sora and Riku look younger due to the dream.
-They have to fight each of their main antagonists in the game
-Xehanort may come back in the dream world, and they fight him there, only to release him into the real world (Prettyyyy sure he's back since the rest of the org. is back.)
-There may be a time limit for how long you can stay in the dream.
-Dreams may be where Ventus's heart is trapped, though I think it's confirmed that it's elsewhere.
Changed it up a little and added some notes.