They can fit in through relationships. A merman OC could befriend students or teachers at Hogwarts. Maybe a merman forms a bond with Hagrid, who is known for his love of all creatures. This friendship could lead to adventures where the merman's knowledge of the underwater world and Hagrid's knowledge of the Forbidden Forest combine in unexpected ways.
One interesting merman OC could be a merman who has a special connection to the Black Lake at Hogwarts. He might be a guardian of sorts, protecting the magical creatures in the lake from dark wizards. His appearance could be a blend of traditional merman features like a fish - like tail with unique magical markings.
An OC can fit in as a new ally or enemy. For example, if the OC is a vampire hunter, they would be an enemy to the Originals. But if they are a witch offering help against a common threat, they could be an ally.
They can also have a connection to the vampire lore in the Lost Boys universe. Maybe the OC has studied vampires in secret and is eager to prove their theories when they meet the real deal. This way, they can interact with the characters in a more meaningful way and contribute to the story.
The OC could be an apprentice to one of the magical characters in the Mary Poppins universe. Let's say the OC is apprenticing under Bert. They learn how to do all his odd jobs and also pick up some of his charm and showmanship. When Mary Poppins returns, the OC is in the middle of learning a difficult dance routine from Bert. MaryPoppins then helps the OC master it, and together they use the dance to break a spell that has been cast over the local theater.
OC can fit into the world - building by being from a new planet or species. For instance, if they are from a planet that has a unique culture based on music, it can add a new aspect to the diverse universe of Guardians of the Galaxy.
The God OC could be an ancient being that has been slumbering for ages. In the fanfiction, something could awaken this God OC, like the excessive use of dark magic. Once awake, they start interfering in the affairs of Westeros. They might side with certain houses based on their own motives. For instance, if they value honor, they might support House Stark. Or they could be a more neutral force, only interested in maintaining a balance of power in the world. They could use their divine powers to stop any one house from getting too powerful, like preventing the Lannisters from completely dominating the Seven Kingdoms.
He could be a henchman who has a change of heart. Maybe he starts off working for one of Harley's enemies but realizes that Harley's brand of chaos is more appealing. He could then become her ally, helping her with her schemes and providing muscle when needed.
The OC gamer could fit in by bringing a fresh perspective. For example, he might introduce new technologies or ways of thinking based on his gaming experiences. With Raynare, he could start by trying to understand her motives. Maybe he offers her a deal where she helps him navigate the world in exchange for something she wants. This way, he can gradually build a relationship with her and find his place in the world.
The male OC could be a skilled fighter. In Daenerys' world full of wars and battles, a strong warrior by her side would be valuable. He could lead her armies, train her soldiers, or be part of her personal guard. If he has a sense of honor and justice, he would be a great addition to her cause. Additionally, he could be an inventor or a craftsman. He might create new weapons or improve the defenses of her cities, which would help Daenerys in her bid for the Iron Throne.
They could be a new recruit to the Cadmus Project, either willingly or unwillingly. If willingly, they might be misled about the true nature of Cadmus. If unwillingly, they are forced into the project. Their journey within Cadmus can be full of challenges and discoveries that lead them to either support or oppose the project.