Yes, there are. You can find some on fanfiction websites like Archive of Our Own. Just search for 'knuckles and rouge love' and you'll get a bunch of results.
In fanfiction, Knuckles is often depicted as a strong and stoic character. He's known for his loyalty to his duties, like guarding the Master Emerald. Rouge, on the other hand, is usually portrayed as a sly and cunning character, with her thieving skills being a prominent part of her character. Their interactions can range from friendly banter to tense rivalries, depending on the plot of the fanfiction.
A common theme is the 'opposites attract' concept. Since Knuckles is a guardian and more straightforward, while Rouge is a jewel thief with a bit of mystery. Another theme is the idea of trust - building. They have to overcome their initial differences and learn to trust each other. For example, in some stories, Knuckles has to trust Rouge not to steal precious items he's guarding.
You could also try FanFiction.net. There are often some hidden gems among the fanfictions there. Look for ones with high ratings and good reviews. A lot of the stories might play with the idea of their different personalities - Knuckles being more serious and Rouge being a bit more sly - and how that dynamic affects their relationship.
There's 'Rouge and Knuckles: A Forbidden Love'. In this fanfiction, their love is set against the backdrop of their different loyalties in the Sonic universe. It's an interesting take as they struggle to come to terms with their feelings while remaining true to their respective causes.
One popular type is adventure - based fanfiction. For example, there could be a story where Knuckles and Rouge team up to find a lost ancient relic. They have to overcome various obstacles and enemies in their path.
It could be about how they first realized their feelings for each other. Maybe during an adventure, they find themselves relying on each other more and more, which eventually leads to their marriage.
The potential for growth in their relationship. Knuckles is a bit of a stoic character, and Rouge is full of charm and guile. In fanfiction, writers can explore how Knuckles learns to open up and show his softer side because of Rouge. And Rouge can find a sense of loyalty and true connection with Knuckles that she may not have had before. Their relationship can be a journey of self - discovery for both of them, which is really appealing in fanfiction.
A sense of isolation is sometimes a theme. Rouge may feel isolated as a vampire, different from her previous self and those around her. And Knuckles may also feel a sense of isolation in trying to deal with Rouge's situation on his own, without fully understanding the vampire world. This creates a complex emotional landscape in the fanfictions.
A common theme is the clash of their personalities. Rouge is sly and often self - interested while Knuckles is noble and duty - bound. Their relationship development is also a big theme, whether it's friendship or something more romantic.