Today we have some confirmed information of some previous speculations:

To unlock Theatre Mode, both Re: Chain of Memories and Kingdom Hearts II: Final Mix+ must be beaten. It is possible to view the cutscenes shown in Kingdom Hearts II in Japanese or in English voices, but the additional scenes DON’T have that option. (I presume this means we won’t have a way of seeing them with English subtitles neither.)

Completing a puzzle in Piece Collection Mode is necessary for unlocking a cutscene in Japanese, without the English option, in Theater Mode.

From the VJump article, it is confirmed that Sora is wearing a crown and not that it is behind him. It is related to the Crown Quest in some way.

The “Card Duel” mode is added at the start of Riku’s Story. Once a “Card Duel” is defeated and “Successful!” is shown across the screen, the outcome of the battle appears on the left hand side of the screen. In a “Card Duel”, it’s most impressive when multiple stock technique cards are used together. However, VJump believes that it seems that the most common number cards in a deck appear less often. The cards that break the enemies’ cards’ attacks are the most important though. After a “Card Duel”, you win some cards, but to win more cards, a play has to think strategically and offensively.

Not really something we didn’t already know, but the new project that’s being created along with Disney will be announced this year.