One common theme is power. How the Naruto - Hellboy descendant deals with their unique powers that come from both worlds. Another could be identity, as they are a part of two very different universes. And of course, there's often the theme of fighting against evil, just like in both the Naruto and Hellboy original stories.
Well, in these fanfictions, a frequent theme is the clash of cultures. Naruto's world has its own set of values, ninja clans, and jutsu, while Hellboy has a very different, more dark and supernatural setting. So the character who is the descendant has to navigate this clash. Also, themes of redemption can be present, especially if the character has some of Hellboy's more 'demonic' traits that they need to come to terms with. There's also the theme of family, as the idea of being a descendant implies a connection to a family line in both universes.
There are definitely some 'Naruto Descendant of Hellboy' fanfictions. However, the quality can vary greatly. Some may have great character development, while others might just be a quick and fun read with lots of crazy mash - up ideas. You could also check out fanfiction communities or forums where people discuss and recommend such works.
One common theme is the struggle with identity. Since the character is a descendant of Naraku, they often have to deal with the stigma associated with that and figure out who they are beyond their heritage.
One common theme is the clash of different mythologies. When Hellboy crosses over with other universes, like the Norse mythology - based Thor universe, you get a mix of different supernatural elements. Another theme is teamwork. For example, when Hellboy teams up with characters from other franchises, they have to learn to work together despite their differences.
Another theme is the exploration of relationships. Female characters in these fanfictions often form different kinds of relationships with Hellboy or other characters from the Hellboy universe. It could be a friendship, a mentorship or even a romantic relationship in some cases.
One common theme is sacrifice. Given their dangerous line of work, a lot of fanfics will have one or both of them sacrificing something for the other. Also, the theme of acceptance is there. Hellboy's appearance and Liz's powers are things that they might have to accept in themselves and in each other more fully in fanfiction stories. And of course, there's the adventure theme as they often go on quests together in these stories.
One common theme is the discovery of self - worth. The oc often starts out feeling out of place or unimportant, but through their interactions with Hellboy, they realize their own value. For example, they might find that their unique skills or personality traits are exactly what is needed to help in a particular situation.
The theme of adaptation is also prevalent. Characters from both sides have to adapt to the new and strange world they find themselves in. For instance, the heroes in My Hero Academia might have to adapt to the more gritty and dangerous world of Hellboy, and vice versa. This can lead to character growth and new relationships being formed.
One common theme is their friendship. Their different personalities - Hellboy's brashness and Abe's more reserved nature - often lead to comical interactions. For example, when they are on a mission together, Hellboy might rush in while Abe is still making a careful plan, and the chaos that ensues can be really funny.
A common trope is the discovery of Naruto's Merlin heritage. It often involves some sort of mysterious event or character that reveals this hidden part of his identity. Another trope is the use of magic in battles. Naruto will use his newfound magical abilities to gain an edge in fights against his enemies, much like how he uses his ninja skills.
Power is a major theme. The Broly - like power in the Naruto world is something that is both a blessing and a curse for the descendant. They have to learn to control it or risk being consumed by it.