Obviously, Luke Skywalker is one of the main characters. He's the central figure around whom much of the story revolves. Rey is also a main character as she has a significant connection with Luke. And then there's Kylo Ren, whose actions in the past are related to Luke's current state, and his presence still looms large in the story.
Well, one key point is Luke's struggle with his own identity and his place in the Force. He has to face his past and his failures. Another is his relationship with Rey, as he becomes her reluctant mentor. Also, there are the battles that take place, which are related to the larger struggle between the Resistance and the First Order.
Naruto would initially be confused by the Jedi Code. His world is so different from the Star Wars universe. But as he spends more time with the Jedi Master, he would start to understand. For example, the idea of not using the Force for personal gain would be new to him. He would have to learn to use his newfound Jedi powers in a selfless way. He might also find it hard to let go of his dreams of becoming Hokage in a sense, as the Jedi Code doesn't really have an equivalent goal, but he would try to find a new purpose within the Jedi teachings.
Ahsoka's training could greatly enhance Luke's skills. She has her own unique combat and Force knowledge. For example, her experience with different forms of lightsaber combat can be passed on to Luke. This may help Luke become more versatile in battles and also deepen his understanding of the Jedi ways.
In the 'The Last Jedi' novel, Luke's relationship status is single. His focus is more on his role as a former Jedi, his past failures, and his attempts to come to terms with the state of the galaxy. There are no romantic relationships or marriages depicted for him in this story. He is more concerned with the bigger issues at hand such as the rise of the First Order and the state of the Force.
In fanfiction, when Luke Skywalker meets the Jedi Council, he might be put to various tests. Maybe they'll test his knowledge of the Force or his combat skills. They could also question his motives and his loyalty to the Jedi ways.
There could be an interesting story where Harry Potter, in this fanfiction, is a Jedi Master Fay. Maybe he uses his wizarding skills combined with Jedi powers. For example, he could use the Force to enhance his spell - casting, making his spells more powerful and far - reaching. He might also use his wizarding knowledge of potions to create substances that could help in Jedi - like training, like elixirs that boost physical strength or mental focus in a way similar to how the Force is used in the Star Wars universe.