One possible plot could be that Sora from Kingdom Hearts accidentally ends up in the Bleach universe. He teams up with Ichigo to fight against a new threat that is causing chaos in both their worlds. They have to combine their unique powers, like Sora's Keyblade skills and Ichigo's Soul Reaper abilities, to defeat the enemy.
The different settings also pose a problem. Kingdom Hearts has various Disney - inspired worlds and its own original worlds, while Bleach is mainly set in Soul Society, Hueco Mundo, and the human world. Merging these settings in a believable way requires a lot of creativity. For example, how would a character from Bleach react to a world like Agrabah from Kingdom Hearts? It's not easy to make it seem natural.
One popular 'Kingdom Hearts and Bleach crossover fanfic' is 'Soul Reapers in the Keyblade War'. It has an interesting plot where characters from Bleach get involved in the Keyblade War scenario of Kingdom Hearts. The author does a great job of blending the two universes' magic systems and character abilities.
A possible plot is that the Soul Reapers discover the existence of Kingdom Hearts. They might think it's some sort of powerful spiritual object. Rukia could be the first to encounter a Keyblade wielder like Sora. Sora, in turn, could be intrigued by the Soul Reapers' powers and their way of dealing with the afterlife. The two groups could then join forces to stop a common enemy that threatens both their worlds. Maybe an ancient evil that wants to use the power of Kingdom Hearts and the souls in the Bleach universe for its own dark purposes.
One of the most interesting pairings could be Yhwach and Ansem. Yhwach has his goal of changing the world in Bleach, and Ansem is obsessed with the power of darkness in Kingdom Hearts. They could form an alliance based on their mutual desires for power. Also, Tatsuki and Naminé. Tatsuki is a strong - willed human in Bleach, and Naminé has her special abilities in Kingdom Hearts. They could develop a friendship where Tatsuki protects Naminé while Naminé uses her powers to help Tatsuki understand the strange new world she's in. And Renji and Terra. Renji with his loyalty to his friends and his zanpakuto skills, and Terra with his struggle between light and dark and his Keyblade mastery, would make for a great pair.
Ichigo and Sora would be an obvious and interesting pairing. Ichigo is a strong - willed Soul Reaper and Sora is a determined Keyblade wielder. Their different fighting styles and personalities would create great chemistry. They could learn from each other, like Sora teaching Ichigo about the different worlds he has visited, and Ichigo showing Sora how to better handle spiritual energy.
I really like 'Bleach - Kingdom Hearts: A New Alliance'. It has an interesting take on how the powers from both series would mesh. For example, it shows how a character from Bleach might use a Kingdom Hearts - inspired weapon in a creative way. The story also delves into the backstories of some characters more deeply, which adds a lot of depth to the fanfic.
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.
There are quite a few. One interesting one might be on FanFiction.net. You can search using the tags 'Sonic' and 'Kingdom Hearts' to find crossovers that combine the high - speed, energetic world of Sonic with the complex and magical Kingdom Hearts universe. It's always fun to see how authors blend Sonic's speed and his friends' abilities with the Keyblade - wielding heroes of Kingdom Hearts.
One great Kingdom Hearts Terra crossover fanfiction is 'Terra's New Journey'. It takes Terra to a different world where he has to face new challenges and make unexpected alliances. The story really dives deep into Terra's character, exploring his strengths and weaknesses in this new setting.