One possible plotline could be that they meet in a dangerous situation, like a battle - ridden landscape. Plague Knight might be initially suspicious of Mona, but as they face challenges together, they start to develop feelings for each other.
I'm not sure of specific 'Plague Knight x Mona' fanfics off - hand. You could try searching on popular fanfic websites like Archive of Our Own. There might be some interesting stories there written by fans who are into this pairing.
Perhaps it could be a fanfic where they meet during a grand magical event. Plague Knight is initially distrusted due to his association with plagues, but Sona sees the good in him. They then team up to stop an evil force that is threatening to disrupt the harmony of their world. The story could explore how their different powers complement each other.
In the fanfic, their relationship could develop in stages. Initially, it could be a relationship of necessity. They need each other to achieve a certain goal, like saving a kingdom from a plague or an otherworldly threat. As they spend more time together, they start to understand each other's quirks and values. This understanding could lead to a deeper emotional bond, whether it's a strong friendship or a more romantic connection. For example, Plague Knight might learn to be more gentle and considerate from Sona, while Sona could gain a sense of adventure from Plague Knight.
One idea could be that they meet in a neutral magical realm. Plague Knight is intrigued by Sona's unique musical magic and Sona is curious about Plague Knight's alchemical skills. They start to exchange knowledge, which gradually turns into a deeper connection.
Another place to search could be FanFiction.net. It has a large collection of fanfictions. However, the quality can vary a great deal. When you search for 'Plague Knight x Sona' there, you might have to sift through some stories to find the ones that really appeal to you. Some might be more focused on action, while others could be more about the relationship development between the two characters.
I like 'A New Dawn for Kaname and Zero'. In this fanfic, it focuses on how they both have to overcome their own inner demons. Kaname's past and his responsibilities as a pureblood vampire are explored, and Zero's struggle with his own vampire side is also a central theme. Their relationship develops in a very natural way as they support each other through these difficult times.
The appeal lies in the contrast between the two characters. Zero is often seen as brooding and conflicted, while Kaname is more mysterious and powerful. A fanfic that pairs them can explore the chemistry that might exist between such different personalities.
One great 'Moon Knight x Reader Fanfic' is 'Midnight Encounters'. It has a really interesting plot where the reader character stumbles upon Moon Knight during one of his nightly patrols in a rather spooky alley. The chemistry between the two is built up slowly and realistically, making it a very engaging read.
Since we lack the details of the 'Mona Darling full story', it's difficult to define who Mona Darling is precisely. She might be a protagonist in a love story, facing challenges in her relationship. Or she could be a strong - willed character in a thriller, fighting against some sort of danger. There are countless possibilities depending on the nature of the story.