Yes, it can be. Combining the worlds of Naruto and Attack on Titan through a fanfiction with Naruto and Mikasa Ackermann can attract fans of both series. People like to explore how characters from different universes interact, and this crossover offers a chance to see Naruto's ninja skills alongside Mikasa's combat abilities in unique storylines.
In Naruto x Mikasa Ackermann fanfiction, a frequently seen theme is the adaptation to a new world. For example, if Mikasa somehow ends up in Naruto's world or vice versa, how they adapt to the different rules, cultures, and people. Another theme is the combination of their combat skills. Writers might explore how Naruto's Rasengan could be used in conjunction with Mikasa's acrobatic fighting style. And there's also the theme of understanding different value systems. Naruto values friendship and the will of fire, while Mikasa values protecting her loved ones. How they come to understand and maybe even adopt some of each other's values is often explored.
It's hard to say. 'Levi x Eren' fanfics are quite common in certain fandoms, but the 'Levi is a criminal' aspect makes it a more specific and perhaps niche sub - genre. If there are a lot of fans who like to see dark or morally ambiguous takes on the characters, it could potentially be popular within that subset of the fandom. However, it also might not be mainstream popular as it goes against the more typical heroic portrayals of the characters.
In the 'Attack on Titan' series, there is no clearly defined canon romantic interest for Levi. His character is mainly focused on his role as a soldier and his fight against the Titans. However, many fans like to ship him with various characters in fan - made love stories.
Maybe Levi is a criminal in the sense that he's part of an underground rebellion against an unjust regime. Eren, who is initially on the side of the establishment, meets Levi and gradually comes to understand his motives. Their relationship could develop as Eren has to decide whether to side with Levi or stay loyal to the regime, all while a romantic connection forms between them.
In the Levi King story, Levi King might be a very interesting character. He could be a person with a special talent. For instance, he might be an excellent musician who uses his music to bring joy to people or to tell important stories. Or he could be a great athlete, competing in some sort of challenging sport and showing great sportsmanship.