One common antagonist could be Klaus. He has his own motives and often clashes with Elijah. His presence can create a lot of tension between Elijah and Elena as well, since he might target Elena to get to Elijah or vice versa.
Elena is often paired with Elijah in fanfiction. Their relationship in the show was complex and in fanfiction, authors explore different aspects of it, like a romantic relationship that could have been. Another character is Katherine. Since she has a long history with the vampires in the show, she also makes an interesting pairing with Elijah in fanfiction.
A fanfiction could explore the idea that Elijah's desire for Elena is part of a bigger prophecy. Maybe there's a hidden message that says if Elijah and Elena are together, it will bring peace to the vampire and human world. So he's driven by this sense of duty as well as his personal feelings for her. This would lead to a lot of internal conflict for Elijah as he tries to balance his feelings with the potential consequences of their relationship.
One idea could be that Elijah tries to court Elena in a very old - fashioned and gentlemanly way. He might take her on trips to historical places he has known over the centuries, sharing his stories and knowledge with her. This would be a contrast to the other vampires' more aggressive approaches towards Elena.
One common theme is rivalry turned friendship. They start off as enemies but over time, in many fanfics, they learn to respect and even rely on each other.
There's a fanfic called 'Elijah's Redemption through Damon'. This one dives deep into Elijah's character development with Damon as an unexpected catalyst. It shows a different side of their relationship, more of a mentor - mentee kind of bond at times.
Well, there is a fanfiction called 'Elijah: The Unseen Years'. This story explores the parts of Elijah's life that we don't see in the show. It might show his travels, his encounters with other vampires or supernatural beings during those times. And also, 'Elijah and the New Alliance' is quite interesting. It's about Elijah forming an unexpected alliance with some of the characters in the series to face a common threat.
One interesting fanfiction could be about Elijah trying to redeem Katherine. Maybe he sees a side of her that others don't, and he attempts to guide her towards a more positive path. It could involve them traveling together and facing various challenges from the vampire world.
One interesting storyline could be Elijah protecting Elena from other powerful vampires. Since Elijah has a sense of honor, he might see it as his duty to keep her safe.
Another reason is the freedom it gives to the writers. In fanfiction, they don't have to stick to the exact rules and storylines of the show. They can bend and break the rules a bit. For 'The Vampire Diaries Elijah OC fanfiction', this means that they can create a story where the OC has a deep, emotional connection with Elijah that goes beyond what we saw in the show. Maybe the OC can understand Elijah's inner turmoil in a way that no other character could. This kind of exploration of character relationships is very appealing to fans, which is why it's so popular.