Another could be an OC who is a foreigner trying to make a place for themselves in the Saxon - Danish world of 'The Last Kingdom'. They might face discrimination but also bring new ideas and skills, like a different style of combat or farming. For example, they could introduce a new type of crop that could thrive in the local climate and change the food situation in a small village.
There could be many. For example, some fanfictions might focus on alternate endings for certain characters. Maybe a character who died in the original story gets a new lease on life and goes on to achieve great things.
One interesting storyline could be a character from our world getting transported into the Fairy Tail kingdom and having to learn magic to survive. They could form unexpected alliances with the guild members and go on adventures together.
Some fanfictions might have a storyline where the worlds of 'Kingdom Hearts' and 'Frozen' start to merge in unexpected ways. This causes chaos as the rules of both worlds collide. The characters then have to find a way to separate the worlds while also dealing with the internal struggles of the newly combined characters. For instance, what if Sora got some of Elsa's ice powers and had to learn to control them while still using his keyblade? This kind of unique situation can lead to very engaging and complex storylines.
In some persona 4 oc fanfiction, an OC might be a detective who is secretly investigating the strange happenings in Inaba. This OC could have their own set of skills and resources, and as they dig deeper, they discover connections between the real world and the TV world that the main characters haven't noticed yet. Their investigations could also put them in danger and lead to some intense action scenes.
Another could be an OC with the Omnitrix getting stuck in a parallel universe where the rules of the Omnitrix are different. For example, instead of being able to transform into known aliens, the OC has to figure out how to use new and strange alien forms that are native to that universe. This could lead to all sorts of adventures as the OC tries to find a way back home.
Another could be a love story within the context of the royals. An original character might fall in love with a royal heir. There could be many obstacles such as differences in social status and the expectations of the royal family. But their love overcomes all these difficulties in the end.
One interesting storyline could be about an original character (OC) who is a half - elf with unique magic abilities trying to find their place in the war - torn world of Warcraft. They might struggle between the two sides of their heritage.
A really engaging storyline might be the OC having to train a young rookie. They take this newbie under their wing, but the rookie has some bad habits that the OC has to break. It's a story of mentorship, growth, and the clash of different generations in the WWE world.
One interesting storyline could be an OC (original character) who is a spy behind enemy lines. They have to use their wits and unique skills to gather intelligence without getting caught.
One interesting storyline could be the nanny getting involved in a local community event in the OC. Maybe she helps organize a charity fundraiser and in the process, she forms new friendships and overcomes some personal insecurities. Another could be her relationship with the family she works for evolving in unexpected ways, like she starts teaching the kids some unique skills that the parents are really impressed by.