
Having this property be the new world for Kingdom Hearts Unchained X makes sense for a variety of reasons...
1) It still fits in with the Fairy tale theme. Every Kingdom Hearts world featured so far has had a Fairy tale theme. The Black Cauldron is as by the book of a Fairy Tale world as you can get. There's a hero, a villain, a Princess (with no Kingdom to rule over) and a Jar Jar Binks.
2) It fits with Kingdom Hearts X dark theme... from a broader perspective, there's going to be a split from X's and Unchained X's storyline. That's guaranteed. What else is guaranteed is that there's going to be events that are far more darker than what we're currently witnessing, and, having a dark fairy tale like this world provides a thematic connection.
3) Square Enix have used obscure Disney properties before. Remember Mickey, Donald and Goofy: The Three Musketeers? or the Mickey short, Runaway Brain? well, they are as obscure as you can get yet they made it into Kingdom Hearts because Square Enix made the creative decision to add that property in. The Black Cauldron is far more recognised than those two properties because it's a feature length film so it's not out of the question despite the original movies financial returns.
Most people are drawing to the conclusion that The Jungle Book is going to be the world featured, however, there's two things that prevent me from believing that The Jungle Book is going to be the "surprise world". Firstly, The Jungle Book's story does not thematically fit as a Fairy Tale world. Yes, it's based of a tale, however, it's not a fantastical tale with magic and daring adventures, it's a coming of age tale. But more importantly, I feel as though having another human in that world just wouldn't work. Mowgli is the "mancub" of that world. Kingdom Hearts III can pull off The Jungle Book because Sora can transform in different worlds. I highly doubt our customisable avatar will "transform".