I'd say it's good. The manga has a complex and engaging storyline that keeps you hooked. The art style is also distinctive and adds to the appeal. But be prepared for some intense and emotional moments.
One common theme is the exploration of the characters' powers in new and combined situations. Since both Naruto and Elfen Lied have characters with unique abilities, fanfiction often delves into how these powers would interact if the characters were in the same universe.
One way is to create a common threat. For example, a new powerful enemy that neither Naruto's ninja world nor Lucy's world can handle alone. This forces them to team up. Another way is through a portal or some kind of magical/scientific accident that brings them together. Then, the story can unfold as they learn about each other's powers and worlds.
Another option could be to check out some Naruto or Elfen Lied - focused forums. There might be fans who share or recommend fanfictions involving both series. However, make sure to follow the rules of the forum when searching for and discussing such content.
Lucy is a very important character. She is a Diclonius with a tragic past. Her powers are both a curse and a means of survival. Kouta is also important. He has a connection to Lucy's past and his kindness towards her, despite her nature, is a key part of the story. Nana, another Diclonius, plays a significant role as well. She has her own journey of self - discovery and her relationship with the other characters is complex.