The struggle between free will and destiny is a big theme. Angels in the 'Supernatural' universe are often bound by their angelic nature and orders from above, but in fanfiction, they may fight for their own free will and try to shape their own destinies.
Love and loyalty are also prevalent. Angels can form deep bonds of love and loyalty, whether it's towards other angels, humans, or a cause. For example, an angel might fall in love with a human and have to choose between heaven and that relationship.
One possible theme could be motherhood itself. Despite being in a supernatural context, the idea of a mother - figure for angels can explore the universal aspects of nurturing, protection, and the bond between a mother and her children. Another theme could be power and its implications. As the mother of angels, she would likely possess great power, and the fanfiction could examine how she uses it, whether for good or for ill.
In 'supernatural fanfiction', a frequent theme is the alternate universes. Writers like to imagine different versions of the 'Supernatural' world, where things happened differently. Romance is also a big theme, whether it's between the main characters or new characters introduced in the fanfic. Also, the origin stories of the supernatural beings are often explored, giving more background and depth to the existing lore.
In such a fanfiction, the relationship between the mother of angels and other supernatural beings would be an interesting aspect to explore. She could be in conflict with demons, or have a complex relationship with humans. Maybe she is a protector of the angels, guiding them in their celestial duties, or she could be a more mysterious figure, whose true intentions are not immediately clear.
One common theme is power. Different bloodlines have different powers, and often there are power struggles or characters trying to control or harness their own powers. Another theme is identity. Characters are often trying to figure out who they are based on their bloodline heritage.
One common theme is the exploration of unknown powers. Sasuke and Naruto often find themselves with new, supernatural abilities that they need to figure out. Another theme is friendship in the face of danger. They have to rely on each other in the dangerous supernatural situations.
A common theme is the insignificance of humanity. In Cthulhu stories, humans are often just tiny specks in the face of the vast and powerful cosmic entities. Another theme is the corruption of the mind. Characters are often driven mad by the mere knowledge or proximity of Cthulhu and his ilk.
One common theme is the clash between military order and supernatural chaos. The soldier has to use his military training to deal with unpredictable supernatural situations.
One common theme could be the vulnerability of the characters. Diapers can symbolize a state of helplessness or a new level of caretaking. For example, if a character is injured or cursed, it might lead to a situation where they need diaper - like care, which adds an interesting dynamic to the relationships between characters in the Supernatural universe.
One common theme is redemption. Since Adam was sort of left behind in the show, many fanfictions focus on him redeeming himself or finding his purpose. Another theme is family. Adam trying to fit into the Winchester family dynamic is often explored.
Identity is another big theme. Nephilim are a unique blend of human and angel. Fanfictions often show them trying to figure out where they belong. Are they more human or more angel? And how do they fit into the world of the supernatural which has its own rules and hierarchies? This exploration of identity can be really interesting and complex.