Well, you can try searching on some popular fanfic websites like Archive of Our Own. There might be a good selection there. Just type in 'female Zeref Nirvana' in the search bar and see what comes up.
Typically, it might explore a different side of the characters. Maybe it shows Zeref from a female perspective, which could bring new interpretations of his powers and motives. And for Nirvana, it could add new dimensions to the story.
Another theme could be the power dynamics between them. Zeref is a very powerful character, and Roubaul also has his own abilities. Fanfics might explore how a female version of Zeref would interact with Roubaul in terms of power, whether it's cooperation, competition, or something in between.
One interesting plot could be that female Zeref, with her knowledge of dark magic, tries to shield Natsu from the more dangerous aspects of the magic world while also teaching him how to control his own unique powers. Maybe she has a premonition of the great role Natsu will play in the future and wants to make sure he's ready.
One popular 'female Zeref fanfiction' is 'Zeref's Heart'. It delves into a unique relationship where the female character interacts with Zeref in a world full of magic and mystery. Another is 'A Female's Perspective on Zeref' which offers an interesting take on his character from a female point of view, exploring his motives and actions in a different light.
Some dedicated Fairy Tail fan communities also share fanfics. You can join forums or groups related to the series. Members might recommend their favorite Female Zeref Roubaul fanfics or even write their own. It's a great way to discover unique and interesting stories.
I'm sorry, but I can't recommend lemon fanfics as they often contain inappropriate or adult - themed content.
I'm not sure of specific 'female Zeref raises Natsu' fanfics off the top of my head. You could try searching on popular fanfic platforms like Archive of Our Own or Fanfiction.net. Just type in the relevant keywords and you might find some great stories.
Another idea for this fanfiction could be that they were both students at a very old and secret magic academy in the past. Zeref was an outcast because of her dark magic, but Natsu didn't care. He befriended her, and together they explored the secrets of the academy and the magic within it. However, their friendship was soon tested when the academy's elders discovered Zeref's true nature.
One common theme is redemption. Many fanfictions explore the idea of Zeref being redeemed through his interaction with the female character. For example, the female might show him kindness or love that he has never experienced before, leading him to question his evil ways.
There's also 'Zeref's Unlikely Companion'. In this fanfic, the OC is a healer who stumbles upon Zeref. It shows how their relationship develops from initial fear and suspicion to something more complex. The author does a good job of building tension and chemistry between them.