One plotline could be that the OC is a mermaid. Killian Jones, being a sailor, has always been intrigued by the legends of mermaids. When he encounters this OC mermaid, she is in danger from a sea - witch. He decides to help her, and in the process, they form a bond. Their relationship could develop as they explore different underwater and on - land adventures together.
In some fanfictions, Killian Jones might be involved in a time - travel scenario. For example, he could accidentally be sent back to a time before he became Captain Hook. He then has to deal with his former self, trying to avoid changing the future while also coming to terms with who he used to be. It could be a really interesting exploration of his character growth.
Love and romance are also very common. The OC and Killian usually end up in a romantic relationship. Writers explore different aspects of their love, like the initial attraction, the challenges they face, and how they overcome them. It can range from a slow - burn romance to a more immediate connection.
One popular plotline could be the challenges they face as new parents. Maybe the baby has some special powers inherited from them, and they have to protect the baby from those who might want to use those powers for evil. Another could be a more comical one where Killian is completely clueless about baby - care at first, but Emma finds his attempts endearing.
One great Killian Jones OC fanfiction is 'The Pirate's New Horizon'. It has an interesting original character who gets involved in Killian's adventures in a really engaging way. The story does a wonderful job of exploring Killian's character further through his interactions with the OC.
One interesting theme is redemption. Killian Jones, with his complex past, often finds himself on a path of redemption in these fanfictions, especially in relation to his interactions with Peter Pan. For example, in some stories, he tries to make up for his past misdeeds by helping Peter protect Neverland.
One idea could be that Killian is on a quest and stumbles upon a young Emma in a magical forest. He initially mistakes her for someone else but soon realizes her potential. Another could be a time - travel scenario where Killian goes back in time and meets a young Emma, and he has to protect her from a dark force without changing the future too much. And perhaps a story where young Emma is lost in a different realm and Killian is the one who finds her and they form an unlikely bond.
One great story could be 'The Enchanted Bond' where Merlin uses his magic to save Killian from a dark curse. It delves deep into their relationship as they face various challenges together in a magical realm.
I would recommend 'The First Meeting'. It's a short and sweet story that gives a good introduction to how Merlin and Killian could meet in an alternate universe. It has simple language and a clear plot.
You could write an OC fanfiction where the OC is a foreign cell that enters Frank's body. This cell has different rules and functions compared to the native cells. It initially causes chaos as the body's systems don't understand it, but Osmosis Jones realizes its potential and tries to work with it to fight off a super - virus that is immune to the normal defenses. The OC could bring in new ways of fighting diseases from the outside world it came from.
One great Killian Jones fanfiction could be the one where he has to face his past in a more in - depth way. It might explore his days as a pirate before he came to Storybrooke. Another interesting one could be about his relationship with Emma Swan in an alternate universe, where they meet under different circumstances. And there are also fanfictions that focus on his growth as a character, from a feared pirate to a hero.