You could make an OC that is a super intelligent animal, like a talking squirrel. This squirrel OC could outwit Brian in various debates about literature or philosophy. It could also befriend Stewie and together they plot some crazy adventures that involve time - traveling or building elaborate contraptions. Or perhaps an OC who is a failed actor and now works at the local bar where the guys from the show often hang out. Their failed acting stories could add a new comedic angle to the bar scenes.
An OC that is a magical being could be fun. For instance, an OC fairy who accidentally lands in Quahog. She tries to use her magic to fix the town's problems but ends up making them worse because she doesn't understand human ways. This could lead to hilarious situations like turning Lois into a toaster or making Peter's car fly uncontrollably. The Griffin family would then have to figure out how to reverse the magic.
One idea could be creating an OC who is a new neighbor in Quahog. This OC could have a really strange hobby like collecting antique doorknobs. They could get involved in the Griffin family's chaos, for example, when Peter tries to borrow a doorknob for some wacky project. Another idea is an OC who is a foreign exchange student at Chris's school. This OC could introduce new cultural elements to the show's typical American setting, leading to funny misunderstandings.
One idea could be creating an OC who is a long - lost relative of the Griffin family. Maybe a distant cousin from another country who comes to Quahog with a whole new set of values and traditions that clash and mesh with the Griffins' way of life. This could lead to some really hilarious and heartwarming situations as the family tries to adapt to this new addition.
Another plot could be that the OC is a time - traveler who accidentally lands in Quahog. The female characters find his stories of different times and places fascinating. They start to form a harem - like group around him as they want to hear more about his adventures. Peter might initially be jealous but then end up getting involved in the OC's misadventures too.
Well, you could write a fanfiction where Meg finally finds true friends who appreciate her for who she is. Maybe a story where Lois discovers a hidden talent and pursues it, causing chaos in the family. Also, a tale about Quagmire having a serious relationship and how it affects his womanizing ways could be interesting.
One idea could be that Dora gets lost in Quahog and has to ask the Griffin family for help. Stewie could be really intrigued by her map and backpack and try to steal them for his own adventures. Another could be that Dora teams up with Brian to solve a mystery in town, like who's been stealing all the cheese from the local deli. Maybe they use Dora's language skills and Brian's detective skills.
One idea could be a crossover where Naruto characters end up in Quahog. Maybe Naruto and Peter Griffin become friends and Naruto tries to teach him some ninja moves. Another could be Stewie somehow getting his hands on Naruto's ninja tools and causing chaos in both their worlds.
Another concept could be a family guy character, like Peter, becoming a ninja in the Naruto world. He would struggle with the strict ninja training because of his lazy and gluttonous nature. But through his interactions with Naruto and the other ninjas, he could learn some valuable lessons about discipline and friendship. For example, he might start off as a joke but gradually earn the respect of the Konoha villagers.
An interesting plot could be an OC who is a noblewoman forced into an arranged marriage with Guy of Gisborne. At first, they clash due to their different personalities and loyalties. However, as they face common enemies and challenges together, they begin to develop feelings for each other. Or an OC who is a thief that Guy catches, but instead of punishing the thief right away, he uses the OC to gather information about other criminals in the area.
One idea could be that Stewie's pregnancy is a result of some wacky science experiment gone wrong in Quahog. Maybe he accidentally drank a potion that made him pregnant. Another could be a time - travel related fanfiction where Stewie travels to a future where male pregnancy is common and he somehow gets pregnant there.
One plot idea could be that the OC guy is a new hero in the game world where Neptune exists. He starts off as a bit of an outcast but Neptune sees his potential. They go on adventures together to save the game world from a new virus threat. As they journey, they start to develop feelings for each other through shared hardships and victories.
A Bat Family OC fanfiction could focus on an OC who is an artist. They use their art as a form of communication and also as a way to leave clues for the Bat Family. This OC might be in the middle of a mystery that involves some of Gotham's art underworld. The Bat Family has to figure out how this OC fits into the bigger picture. It could also be an OC who is a street kid that Batman takes under his wing. This kid has a natural talent for acrobatics and stealth, and slowly becomes an important part of the Bat Family.