An OC could be a former student of Moriarty who has turned away from his evil ways. This OC, with their knowledge of Moriarty's tactics, could be a valuable asset to Sherlock in his battle against the criminal mastermind. They could also struggle with their own past actions and the fear of being dragged back into Moriarty's world, which would create internal conflict in the story.
Perhaps an OC who is a master of disguise and is initially mistaken for a criminal by Sherlock. As the story unfolds, they turn out to be on the same side, helping to solve a complex case that involves international spies. The OC's ability to blend in different environments would add an interesting dynamic to the story.
One idea could be an OC who is a brilliant scientist that crosses paths with Sherlock and Watson while working on a top - secret experiment. The OC's knowledge could either complement or conflict with Sherlock's methods.
Perhaps a plot where Moriarty, Sherlock's nemesis, has somehow made his way into the Star Trek universe and is causing chaos. Sherlock teams up with the Star Trek characters, like Kirk and Spock, to outwit Moriarty. The contrast between Moriarty's devious mind and the more logical and ethical Star Trek characters would create a great dynamic, and Sherlock would be in the middle trying to stop Moriarty's evil plans.
One idea could be that Irene tries to keep her pregnancy a secret from Sherlock at first because she's not sure how he'll react. But then as her pregnancy progresses, she has to rely on him more and more, leading to a new level of closeness between them.
Perhaps in the fanfiction, Joan's pregnancy is a result of a scientific experiment gone wrong. Sherlock then uses his intellect to find a way to ensure the pregnancy progresses safely. They could face opposition from some of their old foes who see the baby as a weakness or a new weapon to use against them. Another plot could be that Joan decides to keep the pregnancy a secret from most people, but Sherlock accidentally reveals it, leading to a series of comical and heartfelt moments.
Another plot could revolve around an OC who is a Spartan trainee that doesn't quite fit the mold. They have a different way of thinking and approaching combat. In Halo 4, they end up being the key to solving a mystery related to the Prometheans. Maybe they find a way to communicate with the Prometheans and convince some of them to stop fighting the humans.
One idea could be an Otsutsuki OC who comes to Earth not for conquest but to study the unique life forms here. They could befriend some of the local ninjas and learn about the concept of friendship, which is alien to the Otsutsuki. This would lead to a series of adventures where they have to protect their new friends from other more hostile Otsutsuki members.
One idea could be an OC who is a long - lost relative of Moana. This OC could have been separated from the family during a great storm years ago. They could have unique skills, like being able to communicate with a different type of sea creature that Moana has never encountered. This would bring new adventures as Moana and the OC try to figure out how this new discovery could help their island.
One idea could be exploring the origin of the OC in the covenant world. Maybe they have a special power that sets them apart from the regular covenant members.
One idea could be an OC who is a new Wolfblood in a pack and has to prove themselves. Maybe they have a unique power that the others don't have and they have to learn how to control it while fitting into the pack's hierarchy.
One idea could be an OC who is a long - lost relative of one of the main characters. They suddenly show up and cause a stir in the power dynamics. For example, if they are related to the main heir, they might have a claim to the inheritance as well, leading to all kinds of family drama and power struggles.
One idea could be an OC who is a brilliant but unorthodox medical student. They constantly clash with House in their diagnostic methods yet prove to be surprisingly accurate at times. For example, they might use a new technology or approach that House is initially skeptical about but later comes to respect.