The relationship between Happy and Fan would be an interesting one. Happy was always very close to Lucy, and with Fan replacing her, Happy might be a bit standoffish at first. But if Fan shows kindness towards Happy and an understanding of his relationship with Natsu, Happy could start to accept Fan. Also, the relationships within the whole guild would shift as members would need to adjust to Fan's presence instead of Lucy's, which could lead to new friendships and rivalries forming.
His relationship with Jiraiya would be different. Jiraiya trained Minato, but with Naruto replacing Minato, Jiraiya might train Naruto even more intensively from an earlier age. Naruto would also probably be more open with Jiraiya about his feelings and plans, as he's not as reserved as Minato.
In terms of her relationship with Gray, there could be a new dynamic. If'si' reincarnated as Lucy, she might be more competitive with Gray, which could lead to some interesting interactions. Her relationship with the celestial spirits would also change. She could become more like a mentor to them, teaching them new things from her past life or experiences. Also, her relationships with other guild members would likely expand. She might form closer bonds with some of the more reserved members of Fairy Tail, bringing them out of their shells.
Her relationship with Natsu would likely change. Natsu might be initially distrustful of Lucy if he senses her connection to End, since End is often seen as an enemy. But then as he realizes that Lucy is still the same person deep down, their bond could become even stronger as he helps her control this new aspect of herself.
The relationships might become more relaxed. For example, characters who are usually a bit reserved like Levy might be more open and friendly with others. Gray and Natsu, who often bicker, could end up laughing together over some drunken joke. And Lucy might be more affectionate towards her friends, giving hugs more freely.
An 'oc' or original character replacing Lucy in 'Fairy Tail' fanfiction can change the whole narrative. 'Fairy Tail' has a large and complex world. By substituting Lucy with an oc, the fanfiction writer can introduce different magical abilities, backstories, and character arcs. It could also be a way to insert themselves or their ideals into the 'Fairy Tail' universe in a more personalized way compared to using an established character like Lucy.
I'm not sure of specific fanfictions right off the bat. But generally, in such a fanfiction, the oc might have different magical powers than Lucy. For example, instead of Lucy's celestial spirit magic, the oc could have shadow - based magic and interact with different characters in the 'Fairy Tail' guild in new ways.
One interesting plot twist could be that Fan has a different magical ability than Lucy. For example, Fan might have the power to control time, which would completely change the dynamics of the story. Instead of using Celestial Spirit Magic like Lucy, Fan could use this time - controlling magic to go back in time and prevent some of the disasters that happened in the Fairy Tail world, thus changing the relationships between the characters and the overall story arc.
His relationship with Lucy would be interesting. If reborn, Natsu might be more protective of Lucy from the start. He could also share his past life memories with her, which would deepen their bond even more. They might start going on adventures together even earlier in the story.
Natsu might be really confused and sad at first. He's used to having Lucy around for adventures. But he would probably respect her decision in the end, knowing it's for the baby.
Lucy and Natsu's relationship could become more intense. In some fanfictions, their bond might develop into a deeper romantic connection, which would change how they interact. Gray might become more of a rival, feeling like Natsu is getting more attention in these fictional scenarios.