One common theme is mentorship. Gildarts being a powerful wizard could be shown guiding Cana in her magical growth. For example, in some fics, he might teach her a new spell or how to control her magic better.
You can try searching on fanfiction - net. It has a large collection of fanfics from various fandoms, and there's a good chance you'll find some gildarts and cana lemon fanfics there.
One good gildarts and cana lemon fanfic could be 'The Father - Daughter Bond'. It explores their relationship in a new light, showing how Gildarts' absence has affected Cana and how they gradually reconnect.
You can try looking on fanfiction websites like Archive of Our Own (AO3). It has a large collection of fanfictions, and you can search specifically for Gildarts and Cana - related ones. Just use the search bar and type in their names.
One idea could be that Cana tries to prove herself worthy of Gildarts' love by going on a difficult quest alone. Another could be that Gildarts, who has always been a bit of a wanderer, finally decides to settle down for Cana. Or perhaps they have to team up against a powerful dark magic that is threatening the Fairy Tail guild and in the process, their feelings for each other deepen.
Definitely include Natsu. His enthusiasm and positive energy can add some light - hearted moments to the story. He could also be the one to accidentally push Cana and Gildarts closer together, like by getting them into a situation where they have to rely on each other. Lucy would also be a good addition. She's a good friend to Cana and can offer a different perspective on love and relationships, having her own experiences in the guild.
The Avengers could become more protective of Tony and his daughter. They would see her as an extension of the team, a new person to safeguard. Black Widow might take on the role of an aunt figure, teaching the girl self - defense skills. And Tony's relationship with Iron Man might change too. He might be more cautious in battles, not wanting to leave his daughter without a father, which could lead to new strategies in fighting villains.
For Merlin, there are both good and bad consequences. On the bad side, some of the more traditional and anti - magic people in Camelot might try to oppose him. However, since Arthur now knows, he has a powerful ally. Arthur can protect Merlin from those who would do him harm. Also, Merlin can now be more open about using his magic for the good of Camelot, which is a positive change.
I'm not sure if 'his wife is story' is a correct expression. It might be a misprint or an unclear statement. If you mean 'his wife is a storyteller', then it means his wife likes to tell stories. If it's something else, more context is needed.
In some fanfictions, Naruto could find out in a very emotional way. For example, his daughter might be in danger, and when he discovers she is his, his feelings of love and the need to protect her are intensified. He might also struggle with the fact that he has missed out on part of her life so far. He'd then start to make up for lost time by spending as much time with her as possible, teaching her ninjutsu and sharing his own life experiences.