One idea could be that the OC is a new transfer student to the school where the characters of Khr Reborn are. They have a special ability that catches Reborn's attention, like being able to sense danger before it happens. This could lead to them being drawn into the dangerous and exciting world of the series.
The OC could be a time - traveler who accidentally lands in the Khr Reborn world. They know some of the future events of the series and try to use this knowledge to their advantage. However, their actions keep changing the course of events, and they have to work with Reborn and the others to set things right while also trying to find a way back to their own time.
One idea could be that the tall OC has a special ability related to their height, like being able to reach high places easily to retrieve important items in the world of Khr Reborn. This could lead to them being recruited by a certain group.
One idea could be a time - travel plot where Tsuna is sent back in time and has to rely on Reborn's teachings from the start again, but with the knowledge he already has. This would lead to some really different interactions and a chance to change certain events in a more strategic way.
One idea could be a story where the OC is a new student at Tsuna's school. They initially clash due to misunderstandings, but as they get to know each other, they start to develop feelings. For example, the OC could be very outspoken and Tsuna is shy at first, but her boldness helps him come out of his shell.
Think about their skills and abilities. If the OC is really good at hand - to - hand combat, it could be because they trained hard to protect someone they love. This motivation can be explored further in the fanfiction. Also, consider how their skills will be used in the context of the Khr Reborn world, like against the various enemies in the series.
One great 'khr hibari x oc fanfiction' is 'A Chance Encounter'. It beautifully develops the relationship between Hibari and the original character. The story has a unique plot where the OC gets involved in Hibari's world in an unexpected way, like helping him during a battle and gradually winning his respect.
One idea could be to create an OC who has a unique power related to the Arcobaleno curse. For example, an OC that can temporarily nullify the curse's effects on the Arcobaleno for a short period. This would add an interesting dynamic as the OC would be both a help and a potential target.
One idea could be that the main character discovers a hidden power within themselves while in the world of 'khr' and this new power is somehow related to love. Maybe it helps them to form stronger bonds with the other characters and overcome a great evil together.
One great khr oc x all yaoi fanfiction could be those that focus on character development. For example, a story where the original character (oc) has to navigate through complex relationships with all the characters in a unique way. The charm often lies in how the oc interacts with different personalities in the yaoi context.
One plot idea could be that the OC is a mechanic who gets caught in the middle of a rivalry between Sideswipe and Sunstreaker. They both start to develop feelings for the OC as she works on their vehicles and helps them in battles. Another idea is that the OC is a human with a special ability that the two Autobots need to protect, and in the process, a love triangle forms. A third idea could be that the OC is from a different planet and has a unique cultural background that fascinates Sideswipe and Sunstreaker, leading to a complex relationship among them.
I'm not really into that specific type of fanfiction, so I can't recommend any particular works. However, you can try searching on some popular fanfiction platforms like Archive of Our Own. They usually have a wide range of works and you might be able to find what you're looking for by using the relevant tags.