Makarov could be a great father figure in Fairy Tail fanfiction. He is already like a father to many in the guild. He has the wisdom and experience, and his protective nature towards the guild members is very father - like. He could be the center of a fanfiction story where he has to make tough decisions to protect the guild family, similar to what a father would do for his children.
Well, if we're following the common ships in the Fairy Tail fandom, Natsu could be a likely candidate. Their relationship is very close and there are many hints of a deeper connection between them in the original series. It would make for an interesting story as Natsu would have to grow up a bit to take on the role of a father.
One potential father figure for Ash in fanfiction could be Professor Oak. He has been like a mentor and a guiding figure in Ash's life. He has given Ash his first Pokémon and has always been there to offer advice, which could easily be developed into a father - like relationship in fanfiction.
Gray could also be a possibility. He and Lucy have a friendly rivalry and there are moments where their relationship could be interpreted as having deeper feelings. It would be an interesting twist if Gray was the father, as it would change the dynamics between him, Natsu, and Lucy in the fanfic.
One idea could be that Elfman and Evergreen are sent on a mission together alone. They start off bickering as usual, but as they face various challenges, they begin to understand each other better. For example, they have to work together to defeat a powerful dark mage. During this time, Elfman discovers Evergreen's insecurities behind her tough exterior, and Evergreen realizes that Elfman is not just a brute but has a kind heart.
In some fanfictions, Erza's father could be a powerful mage who was lost in a great battle against dark forces. His absence then becomes a driving force for Erza's growth and her determination to fight evil.
A great plot might involve Gray and Juvia adopting a child. The child could come from a troubled background and be initially distrustful of them. Gray and Juvia would have to work hard to gain the child's trust. Along the way, they could also teach the child about magic and the importance of family in the Fairy Tail guild. As the story progresses, the family unit grows stronger and they become an inspiration to other members of the guild.
Storm Fullbuster, son of Gray and Juvia. He could have a unique blend of ice and water magic. His story could revolve around him trying to live up to his parents' legacies while also forging his own path in the guild.
Juvia might also be a good candidate. She's deeply in love with Gray already, so the concept of her becoming the end bride could add a whole new layer of drama. Her water magic could be used in unique ways in the context of the end bride story, like creating a magical shield around herself and her loved ones as she steps into the role of the end bride.
Erza could also be a possibility in some fanfictions. She is strong and independent, and Natsu respects her a great deal. A relationship between them would be full of interesting dynamics, like how they would handle their different leadership styles when they are together, and how their relationship would affect the guild as a whole.
It could be a rival guild. There are other guilds in the Fairy Tail universe that might be jealous of Fairy Tail's power and reputation. They could kidnap Mirajane to try and weaken Fairy Tail. For example, Phantom Lord has had conflicts with Fairy Tail in the past.