WMG / The Magic Treehouse

The Treehouse is a TARDIS
  • Its chameleon circuit is also broken but its stuck as a treehouse in a tree.
    • And ergo, Morgan le Fay is a Time Lord.
  • Alternatively, the Treehouse is primitive TARDIS constructed with Arthurian Era materials and magic that Morgan and others would have known about, hence why it looks, inside and out, like a simple treehouse full of books, instead of a futuristic spaceship concealed within the geometric appearance of a simple treehouse.
It is in the same universe as a particular light novel, or is at least the predecessor.
  • There was once a light novel series called Domitor Leo. It spanned six books. The adventures are quite similar in fashion to this series; the only difference is that the books involve a disaster and that the characters are being sent by a particular British person working on the Big Ben clock tower. And the origins of the series' happenings are Arthurian!

Obvious already?

The books have their own special kind of magic.
By that I mean that the books change history. The things that are written in there change things in real life. For example, why are there dinosaurs without feathers in the first book? That's because the book changed history and you see that instead of anything else.

You can only go into the book's contents, but not actually travel in time.
That's why in the first book there's dinosaurs without feathers. Because the book showed them like that, instead of what they really are. That would also explain why, for example, in Eve of the Emperor Penguin, no one made a big fuss on the news about two children suddenly showing up on an Antarctic research base: because they only traveled into the book, not within their own world. This also prevents anyone using the treehouse from changing their own life or, even worse, committing a Grandfather Paradox, because anything they do while traveling in the books will not affect their own timeline.

In-Universe, the versions of Arthurian Mythology where Morgan is evil are an attempt by her enemies to ruin her reputation in history.
Upon first meeting Morgan, Jack says "They said you were a witch.", indicating that tales with a negative portrayal of Morgan exist in this world, and are widespread enough that they're in books read by 8-year-olds. However, from what we've seen in the series, we know that Morgan is actually a Big Good who has helped save Camelot on several occasions and organized the recovery of several powerful artifacts like the Diamond of Destiny and the Wand of Dianthus that might otherwise have fallen into the wrong hands. As Blizzard of the Blue Moon showed, some evil wizards are capable of traveling to the mundane world's past, so it's possible that one of them organized a Smear Campaign against Morgan out of spite. (And/or to encourage people who encountered her or people who claimed to work for her to refuse help. Obviously it didn't work, as Jack and Annie usually have very little trouble finding help when they need it.

Jack and Annie are related to Marty Mc Fly.
Time travel just kind of runs in this family. We do meet Marty's kids in Back to the Future Part II, but Marty did have two siblings. Jack and Annie's mom could be Marty's sister, or his brother could be their dad.

Jack and Annie are descended from a wizard from Camelot.
They have magical abilities, and the treehouse landing in their neighborhood may be due to a family connection.

The next Camelot special will be about giants.
Giants are common figures in European lore, especially Celtic mythology.

The Dark Wizard of Mordred is Sir Meleagant.
Sir Meleagant captured Guinevere to rid Camelot of its joy, similar to how the Dark Wizard of Mordred enchanted Camelot in "Christmas in Camelot." The Dark Wizard being Meleagant would also allow him to appear without risking confusion with the Green Knight. The Dark Wizard will appear soon, seeing as Jack and Annie have faced many of his supporters already.

Kathleen is a Pendragon.
Arthur likely has a lost successor, and as common in epic fantasy, the hero often gets the princess. Kathleen has dark, curly hair, a mix of Arthur's hair color and Guinevere's hair structure. She also is a poor weaver, implying her hands aren't used to the work. "Winter of the Ice Wizard" reveals humans can take on fey aspects if they live with them for a while, explaining Kathleen's selkie abilities. The Diamond of Destiny lights up in Jack's hands, and in "Shadow of the Shark," he is offered the role of Mayan king, possibly foreshadowing him succeeding Arthur.

Or, Annie is a "future" Pendragon.
In some versions of King Arthur, Merlin is picky over Arthur's choice of a queen, doubting Guinevere. In The Magic Treehouse, Merlin takes an interest in Annie, giving her powerful tools such as the stardust she uses in "Balto of the Blue Dawn." Annie also has displayed a fondness for fairytales throughout the series, especially Cinderella. Annie is more involved in the Camelot adventures, showing her interest in the land.
http://tvtropes.org/pmwiki/pmwiki.php/WMG/TheMagicTreehouse