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.
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.
Character interactions are quite unique in Naruto Elfen Lied crossovers. Naruto's optimistic and determined personality against the often more brooding and conflicted nature of the Elfen Lied characters can lead to some intense and thought - provoking exchanges. Also, the idea of the Elfen Lied characters learning about ninjutsu or the Naruto characters learning about Diclonius powers can open up a whole new world of character development.
I'm not sure of a specific title but in general, a good Naruto Elfen Lied crossover fanfic would probably have some really cool fight scenes. Imagine Naruto using his Rasengan against an Elfen Lied Diclonius. It would be amazing to see how the different power systems interact. Maybe there's a fic out there where the main characters from both universes have to team up to fight a common enemy that is threatening both their worlds.
One idea could be exploring the challenges the pregnant character faces in a world filled with the chaos and danger present in the Elfen Lied universe. Maybe dealing with the fear of how the child will be received due to the unique nature of the Diclonius.
First, know the Elfen Lied universe well. Understand the rules, the nature of the characters, especially the Diclonius. Then, develop a strong plot around the pregnancy. Make it more than just about being pregnant. For example, tie it to the overarching themes of the series like discrimination and survival.