I'm not sure if there is a specific 'lucy x sting' fanfic rated M. You could try searching on popular fanfic websites, but make sure to follow their terms and conditions.
There's not enough information to really know who Lucy specifically is. She could be just a regular reader of fanfiction who took the time to rate this piece as M. Maybe she's part of a fanfiction community where people rate works to help others know what to expect.
You can start by looking on fanfic - specific websites such as Archive of Our Own (AO3) or FanFiction.net. These platforms have a large collection of fanfics from various fandoms, so there's a good chance you'll find some 'lucy x sting' fanfics there.
Rated M often indicates that the fanfiction has content that is considered more appropriate for a mature audience. Maybe Lucy, being a reader or a reviewer, felt that the fanfiction had elements such as strong language, intense or disturbing themes that led her to rate it M. It could also be that there were some sexual undertones or implications, but not explicit, which pushed it into the M - rated category according to her.
One common theme is friendship development. They often start as acquaintances or even rivals and then become close friends. For example, in many fanfictions, Sting initially sees Lucy as just another mage but as they interact more, they find mutual respect.
There could be many different fanfictions with Lucy and Sting having a pregnancy plot. However, it really depends on the creativity of the fanfiction writer. Some might write it as a way to develop their relationship further, perhaps adding more drama and complexity to their story.
Another theme is adventure. Many fanfics put them on exciting adventures together, which tests their relationship and makes it stronger. They could be exploring a mysterious magical realm or facing a powerful enemy.
Love and friendship are common themes. Usually, it focuses on how the reader and Sting develop feelings for each other, whether it's a slow - burn romance or a sudden spark. Another theme could be adventure, where they go on quests together.
One common theme is adventure. Often, they embark on some sort of journey together, whether it's to find a lost magical item or to defeat a powerful enemy. This allows for their relationship to develop in the midst of exciting events.
One way to mark rated m fanfic is to use tags. For example, if it's on a fanfic website, you can add a specific 'Rated M' tag to the metadata of the story. This way, when users search or filter, they can easily identify these fics.