First off, think about what kind of story you want to tell in the Kingdom Hearts universe. Maybe it's a new adventure for the main characters or a side story about a minor character. Make sure your writing style matches the tone of the original series and keep the magic and fantasy alive throughout.
It might be seen as a fanfic because it combines elements from various Disney and Square Enix franchises in an unconventional way, appealing to fans' imaginations and creative reinterpretations.
Another area of advanced theory could be related to the connections between different worlds in the Kingdom Hearts universe. In fanfic, authors might postulate new ways that worlds interact, such as hidden portals or a shared energy source that links all the worlds together. This could lead to interesting plotlines where characters discover these new connections and use them to their advantage.
Advanced theory in Kingdom Hearts fanfic gives fans a chance to expand on the existing lore. It allows them to take the concepts from the games and build on them in creative ways. For instance, if the official story only briefly mentions a certain magic spell, fanfic writers can develop an entire theory around it, like how it could be used to traverse between different dimensions.
A Kingdom Hearts fanfic might be considered cumbersome if it has overly complex plotlines. For example, if the author tries to incorporate too many characters from different parts of the Kingdom Hearts universe all at once, it can become confusing. Also, if the writing style is overly wordy and filled with long, convoluted descriptions that don't really add to the story, it can make the fanfic cumbersome.
One common theme is self - discovery. Since the OCs are new to the Kingdom Hearts world, they often have to figure out who they are and what their place is. Another theme is friendship, as they usually team up with the main characters from the game and form bonds. Adventure is also a big theme, as they travel through different Disney - inspired worlds.
The fanfic often plays with the different powers and abilities. For example, the keyblades in the MLP world are a unique concept. The ponies might use them in ways different from how they are used in Kingdom Hearts, adding a new dimension to the story. Also, the magic in MLP and the magic in Kingdom Hearts can be combined in creative ways in these fanfics. They might create new spells or ways of fighting that are unique to this crossover.
First, study both universes thoroughly. Know the characters, their powers, and the overall lore. For example, in Bleach, understand how Soul Reapers' zanpakuto work, and in Kingdom Hearts, know about the Keyblades. Then, create an interesting plot that can bring these two different worlds together in a believable way.
Identity crisis is a big theme. Fem Harry is used to being one thing in the Harry Potter world, but in Kingdom Hearts, she's in a whole different ballgame. So she struggles to find her place. There's also the theme of good vs. evil. In the Kingdom Hearts universe, there are dark forces, and fem Harry has to decide where she stands. Also, the theme of exploration as she travels through different worlds in the Kingdom Hearts realm.