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 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.
Lucy does survive. The manga's ending provides a resolution that leaves her with a chance at a new beginning. There are various factors and plot developments that contribute to her survival.
Definitely not a happy ending. The manga takes some dark turns and doesn't wrap up in a traditional cheerful way. It's more thought-provoking and emotionally charged than just happy.
A plot could involve a male Diclonius who discovers he has a unique power among his kind. He is then hunted by other Diclonius who see him as a threat or a valuable asset. As he tries to evade them, he stumbles upon a hidden community of Diclonius who have different ideals and he has to decide whether to join them or continue on his own path. He could also be on a quest to find out the origin of his kind and the true purpose of their powers.
Well, one key element could be the contrast in powers. Naruto has his ninja skills like shadow clones and rasengan, while Lucy from Elfen Lied has her vectors which are super powerful. Another element is the different worlds they come from. Konoha in Naruto is a ninja - dominated world, and the world in Elfen Lied is more post - apocalyptic and full of mutants. The character interactions would also be interesting. Naruto's positive and determined personality might clash or bond with Lucy's more complex and often misunderstood nature.