When writing a He - Man crossover fanfic, research is key. If you're crossing over with Lord of the Rings, study the races, the magic systems, and the geography of both universes. Develop interesting character relationships. He - Man might form an unlikely friendship with a hobbit, or have a rivalry with an orc. And don't forget about the dialogue. It should feel natural for both the He - Man characters and those from the other universe, so readers can really get into the story.
He-Man's story involved Prince Adam using the power of Grayskull to become the mighty He-Man and engage in epic battles. It was filled with adventure, magic, and the struggle to keep Eternia safe from the clutches of Skeletor and his evil plans.
I'm quite sure there exists a 'Naruto is One Punch Man' fanfic. The world of fanfiction is vast and full of all kinds of creative crossovers. Writers might be intrigued by the idea of bringing Naruto's ninja skills and the unique world of One Punch Man together. For example, how would Naruto react to the overpowered villains in One Punch Man's world? And how would Saitama view Naruto's determination to protect his village? These are the kind of things that could be explored in such a fanfic.
Definitely not. 'He-Man' has nothing to do with Marvel Comics. It has its own distinct brand and fan base that are separate from anything Marvel-related.