First, study Vanitas' character. Know his traits, like his brooding nature and his complex past. Then, come up with an interesting plot. It could be about his past that hasn't been fully explored in the original. For example, his early days. Also, create engaging dialogues for him. Use words that match his personality.
One great 'kh vanitas fanfic' is 'Vanitas' Redemption'. It delves deep into Vanitas' character and his journey towards self - discovery. Another is 'Vanitas in the Shadows' which shows a different side of him in various uncharted scenarios. And 'The Vanitas Chronicles' is also popular as it weaves an interesting plot around Vanitas.
One unique way is to have lemons be the source of a new kind of magic in the Vanitas world. Maybe Vanitas discovers that by squeezing a lemon and mixing its juice with a special powder, he can create a potion that gives him temporary superhuman abilities. Or the lemons could be sentient beings that communicate with Vanitas in a secret language only he can understand, guiding him on his quests and helping him solve mysteries. Another aspect could be that the lemons are from a different dimension and Vanitas has to travel there to understand their true power and how they are related to his own existence.
KH3 has a really engaging story. It's filled with complex characters and a rich world. The story takes you on an adventure through different Disney - inspired worlds, which is really cool. The character development is quite good, especially for the main characters like Sora. It also has a deep underlying plot that keeps you hooked from start to finish.
The Vanitas no Carte manga has come to an end. The creators completed the story and provided closure for the fans. It was a memorable journey throughout.