Well, 'angel on my shoulder fanfic' could potentially be a rewrite or an expansion of the original concept. For example, it could focus on the backstory of the angel. What was the angel's life like before being on the character's shoulder? It could also delve into the moral and ethical dilemmas that the presence of the angel creates for the character.
Maybe the angel on the shoulder has lost its memory and as the story progresses in the fanfiction, the protagonist helps the angel regain its memories while also dealing with their own problems in the human world, like school or relationship issues.
A possible plot is that the character discovers that there are other forces trying to take the angel away from them. So, the story could be about the character's fight to keep the angel on their shoulder, with all the battles and challenges they face along the way. This might involve making deals with other supernatural beings or finding a way to hide the angel from those who want to take it.
There were many novels that were on par with Covering the Sky. Some of the famous novels included Battle Through the Sky, Martial Universe, Full-time Expert, Ghost Blows Out the Light, Douluo Continent, and so on. These novels had a high popularity and reputation. Not only did they occupy a high position in the field of online literature, but they had also been adapted into television dramas, movies, and other forms of works, receiving widespread attention and love.
One common theme is the struggle between good and evil. Since you have an angel representing good and a devil representing evil, there's often a battle of ideologies. Another theme is redemption. Sometimes the devil character is shown to be redeemable through the influence of the angel.
I'm not sure specifically as I haven't read this exact novel. But generally, it might be about a character who appears to be bad (the devil suit) but has good or angelic qualities inside.
Give them common human flaws. For example, if the angel character is always seen as perfect, give it a fear of heights or a small addiction to human food which is considered strange in the angelic world. This makes the angel more relatable.
One common theme is the struggle between good and evil. Since angels are often associated with good and devils with evil, their interactions and the choices they make highlight this eternal struggle.
It could end with separation. Their differences may be too great to overcome, and they are forced apart. Maybe the forces of heaven and hell won't allow their union.
One common theme is the struggle between good and evil within the relationship. The devil represents temptation and chaos, while the angel stands for purity and order. Another theme could be redemption, where the devil may be redeemed through the love of the angel.