Well, a common theme is the supernatural elements. In the 'Black Butler' universe, both Sebastian and Claude are demons, so fanfictions often play around with their demonic powers, like how they use them in different situations or how they hide them from the human characters. Also, there's the theme of temptation. They are always trying to tempt their masters or other characters, and fanfictions might explore what happens when those temptations go too far or are resisted.
One popular theme is the power struggle between Sebastian and Claude. Since they are both highly capable butlers with their own motives, their clashes over protecting their masters or achieving their own goals make for exciting storylines. Another theme could be loyalty. Despite their devious natures, they show a form of loyalty to their respective masters, and exploring the limits and true nature of that loyalty in fanfiction can be really interesting.
Some fanfictions focus on the relationship between Sebastian and Claude outside of their duties. Maybe they form an unlikely alliance for a short time to achieve a common goal. This could be due to a threat that is bigger than their rivalry. Or it could be about the mystery surrounding their pasts. Writers might create stories that dig into their origins as demons, how they became the butlers they are, and if there are any past connections between them.
Chapter 54, "Revised: Chapter 54 Bodies Everywhere [Please Follow!]", is the clearest early answer because it turns Rogel Camp's situation into a concrete plot event. Early on, Luo Ming's willingness to follow Xia Hong back could be considered.... By the end, Let's first return to the camp..
Chapter 123, "Arrow Bamboo Forest, Rat King, New Camp", is the clearest early answer because the buildup finally cashes out in a real clash. It opens with Seeing the actions of the four, a trace of emotion flashed in Xia... and closes on There was no suspense, the tree's surface was pierced by the Yi....
Chapter 116, "Great Righteousness, Hunting Flood Dragons, Saliva [Five-in-One Mega Chapter]", is the clearest early answer because it turns Yuan Cheng's situation into a concrete plot event. It opens with There are three large trees around, just barely reaching Teng Jiao's... and closes on Can this saliva neutralize poison?" Covering their mouth and nose.... The key payoff is that the chapter closes with The Zhao Long brothers at the front didn't use blades.
Chapter 159, "Blood and Flesh Wall, Has the Leader Not Broken Through Yet?", is the clearest early answer because it turns Iron's situation into a concrete plot event. The setup is A few low-level Cold Beasts don't pose much of a problem, with so..., but the pressure keeps building through The key is, what these Cold Beasts can break isn't just the walls of....
Chapter 1, ", The World in the Grip of Winter [Please Add to Favorites!]", is the clearest early answer because it turns Xia Hong's situation into a concrete plot event. The chapter turns that into something concrete by moving from The cold wind raged, and snow filled the sky. to Because, before crossing, he was a patient with ALS..
Chapter 21, "The Great Harvest", is the clearest early answer because it turns Snow Mane's situation into a concrete plot event. The setup is This Snow Mane was actually extremely cautious., but the pressure keeps building through When dealing with the Frost Wolf, even when gravely injured, it did....
Chapter 115, "New Earth-digging Realm, Enhance Mining Efficiency [Three-in-One]", is the clearest early answer because Hearing Xia Hong finally turns buildup into a real advancement. A useful checkpoint is the way the scene starts with Since contributions vary, the emergence of hierarchy and comparison... and follows through with Unexpectedly, Yuan Cheng managed to surpass them..
Chapter 104, "Camp Strength, Rapid Enhancement (2-in-1 Large Chapter)", is the clearest early answer because it turns Iron Ore's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Opening the camp system interface and seeing the data on Wood, Coal,... and follows through with The point is that receiving Xia Hong's personal affirmation fills....
Chapter 484, "Intercepting Attacks", is the clearest early answer because the buildup finally cashes out in a real clash. The setup is The wind howled against Skylar's wings as Damien soared northward,..., but the pressure keeps building through The towering walls, the barracks, the winding streets, and the people....
Chapter 521, "Storming The First Base II", is the clearest early answer because it turns Damien's situation into a concrete plot event. The chapter gives readers a real answer by opening on He had walked less than a kilometer from the abandoned stronghold... and ending on The creature roared and released a shockwave of dark essence outward..