If Luffy ate a different devil fruit, his entire power set and character would change drastically. For example, if he ate a Logia type like the Smoke - Smoke Fruit, his fighting style would be more about evasion and stealth. He could turn into smoke to avoid attacks, and his stretching abilities would be gone. His crew might also have to adjust their battle strategies around his new powers.
Imagine Luffy ate a devil fruit that gave him the power to control minds. This would be a very different power compared to his current Gum - Gum abilities. His crew would have to trust him completely not to abuse this power. There could be ethical discussions within the crew. Also, in terms of leadership, Luffy might have an easier time getting others to follow his plans, but it could also create some tension if the crew felt he was relying too much on his mind - control powers instead of his natural leadership qualities.
It could also be interpreted as a platonic display of deep friendship. In the world of One Piece, the Straw Hat crew has a very strong bond. The kiss might be a way to show that their connection is beyond the normal boundaries of friendship, not necessarily romantic but a very intense form of comradeship. For example, it could be a way to boost each other's morale in a difficult situation.
Another possibility is a Devil Fruit that gives her the power of telepathy. Fem Luffy could then communicate with her crew without speaking, which would be very useful in stealth missions. She could also use it to read the minds of her enemies, getting an edge in battles. This would completely change the dynamics of her relationships with other characters in the story as well.
If Fem Luffy had a different Devil Fruit, she might not have the stretchy body that makes her so recognizable. For example, if she had a Devil Fruit related to plants, she could grow vines from her body to grab enemies or create barriers. This would make her more of a strategic fighter, relying on the environment she can create rather than her physical strength alone.
Well, in canon, Luffy is very straightforward in his interactions with Whitebeard. But Fem Luffy could bring a different energy. She might be more inquisitive about Whitebeard's past and his reasons for doing things. Also, Whitebeard might treat her a bit differently. Maybe he'd be more protective or more cautious around her because she's a female in a pirate world that's often tough on women. This would change the power dynamics and the nature of their conversations.
If Harry Potter was raised by Luffy, his personality would change significantly. Luffy is always cheerful and optimistic, and this would seep into Harry. Harry would be less haunted by his past and more focused on the present and future. His sense of humor would also be different. Luffy has a very simple and direct sense of humor, and Harry would probably adopt some of that. He would be more likely to laugh at the simple things in life. In addition, Harry would be more impulsive. Luffy often acts on impulse, and Harry would be more likely to do the same, which could lead to both great adventures and some trouble. His loyalty would also be more like Luffy's, fierce and unwavering towards his friends.
Another idea is a Devil Fruit that gives Luffy the ability to transform into different animals. He could turn into a powerful eagle to fly over islands and scout for danger or transform into a strong bear to have enhanced strength and endurance in close - combat battles. This would change his fighting style completely and make for some really interesting interactions with his crew and opponents.
Fem Luffy would probably be more worldly. Shanks has a wide range of experiences and connections. Growing up with him, Fem Luffy would have met different people from different backgrounds. As a result, she would be more diplomatic and better at handling various situations. For example, when dealing with other pirate crews or even the Navy, she would know how to communicate effectively. Also, she might have a more refined taste in food and other aspects of life compared to the regular Luffy, as Shanks' crew might have introduced her to different cultures and cuisines.
Sorry, I'm unable to tell who the author of this 'im the king novel' is. It could be an indie author or a relatively unknown writer. You could try searching for it in literary databases or online bookstores to find out more about the author.