Today, with the release of Kingdom Hearts HD 2.5 ReMIX in Japan, Famitsu Weekly have included a special commemorative article in their October 2, 2014 issue. This article contains a detailed summary of the game, teases both of the new Secret Movies included with Kingdom Hearts Re:coded, and also contains a special interview with the director of the Kingdom Hearts series, Tetsuya Nomura.
In this interview, Nomura has revealed that he hopes to one day tell the story of Birth by Sleep -Volume 2-, when he is given the opportunity. Birth by Sleep -Volume 2- was of course the ending "title card" of the Secret Movie for Kingdom Hearts Birth by Sleep Final Mix, leading fans to speculate that this might be a mystery game in the works. However, this movie was released in 2011, and according to Nomura today, "the story was severed and wasn't told".
Read Nomura's full comment below, thanks to KH13 user SkyKeybladeHero for translations.
—Among everything that was included in the FInal Mix version, what would you recommend?
Nomura: The Secret Episode. It's an evolution of all the Secret Movies I've put in the games so far, because it's playable. You can feel the pace of the story and the battle, so I'm glad I made it like that. By the way, the indication for Birth By Sleep -Volume 2- is still there. It shows the existence of Aqua's story in the Realm of Darkness. It means that she didn't just wander about in the Darkness for ten years, but that she passed through many experiences. However, it is a "Fragmentary Passage", as the same suggests-- that story was severed and wasn't told. I hope I can tell it when given the opportunity.