Give the OC a unique skill or trait that is relevant to the Sherlock universe. It could be something like an eidetic memory or a talent for deciphering ancient codes. This way, they can interact with Sherlock and the other characters in a meaningful way, either as an ally or an adversary.
Integrate the OC into existing relationships. They could be an old acquaintance of Watson from his military days, or have a connection to Mycroft through some government work. By tying them into the pre - existing web of relationships in the Sherlock universe, they will seem like a more natural part of the story.
An OC can be well - integrated by having a backstory that ties into the existing lore. For example, if the OC has a family connection to an existing wizarding family or a significant event in the wizarding world's history. Also, their magical abilities should be in line with what we know about magic in the Harry Potter universe. They can't have some completely out - of - the - blue superpower.
To integrate an OC well, first, consider their personality. If the OC is a confident and brash character, they could contrast with Android 21's more complex and often reserved nature. Second, think about their goals. If the OC's goal is to find a cure for a certain disease that Android 21 might be related to in some way, it creates an instant connection and a plot thread. Third, make sure their appearance also fits the DBZ aesthetic, like having a unique fighting gi or a distinct hairstyle.
The oc should have a backstory that ties in with the supernatural world. If it's a world of demons and angels, the oc's past could involve some sort of encounter with these beings that led them to be in this world now. This makes their presence more believable.
A Zoro OC can be well - integrated by having a connection to the existing world of One Piece. Let's say the OC has a dream related to the All Blue, just like Sanji. This connection to an existing concept in the One Piece universe makes the OC feel more at home. Their relationships with other characters are also crucial. If the OC has a friendly rivalry with Zoro, similar to Zoro's relationship with Sanji, it adds depth to the story. They can compete in sword - related challenges and in the process, both grow as characters.
The OC can be introduced as a new recruit in Fubuki's group. For example, they could be a young and inexperienced hero who is in awe of Fubuki's powers and leadership. They start by following Fubuki's orders and gradually prove themselves useful in battles.
The OC can be integrated by having a unique talent related to a cappella. For example, if the OC has an extraordinary beatboxing skill, it can be a great addition to the group's performances in the fanfiction. This would make the OC valuable to the Pitch Perfect crew and give them a reason to be part of the story.
First, give the male OC a believable backstory related to his past life and how it ties into the BNHA world. For example, if he was a scientist in his past life, his knowledge could be used to develop new support items for heroes. Second, have him interact with the existing characters in a natural way. He could start as a transfer student at UA and make friends or rivals with the canon characters. Third, give him a unique quirk that both fits the BNHA universe and is related to his past - life experiences.
The OC can have pre - existing relationships with some of the minor characters in the High School DxD world. For example, they could be an old friend of a side character who then gets introduced to the main group. This way, it feels more natural for them to be part of the story.
The OC's personality should complement or contrast with Deadpool and Cable's. If the OC is very morally upright, it can create great tension and comedic moments when paired with Deadpool's morally ambiguous nature and Cable's more rigid code. Also, giving the OC unique abilities that are relevant to the story's plot, like an ability to heal that is crucial in a battle - ridden fanfic world of Deadpool and Cable.
The OC should have a clear motivation that ties into Tai Lung's world. For example, if the OC is also a martial artist, they could be seeking to prove themselves against Tai Lung, which immediately creates a connection.