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.
It depends on how the author of the fanfiction has developed the plot and the characters. They could have given Natsu some special power - up or a unique situation that makes him stronger than Gildarts. For example, the author might have introduced a new form for Natsu that gives him an edge in combat, or perhaps they've weakened Gildarts in some way for the sake of the story. However, in the original series, Gildarts is extremely powerful, so this is a significant departure in the fanfiction.
There could be. However, it depends on how popular the concept of Naruto being Gildarts' brother is among fanfiction writers. Some concepts are more common than others in the fanfiction world. You may need to dig a bit deeper into the fanfiction communities or specific Naruto - Fairy Tail crossover sections to find it.
One interesting storyline could be Cana's journey to become a more powerful mage. Maybe she discovers a new type of magic or a long - lost spell. Another could be her relationship with the other guild members, like a deeper friendship with Lucy or a rivalry - turned - friendship with Erza. There could also be fanfictions about Cana's past, exploring her family history and how it relates to her magic abilities.