There might be a fanfiction where Arataka Reigen somehow gets involved in the Akatsuki's plans. Since he is so good at bluffing, he could try to infiltrate the Akatsuki and end up in all sorts of crazy situations. Or perhaps a story where he helps Naruto train his Rasengan in a really strange way, like using his psychic - like skills (even though they are mostly fake in his original series) to enhance Naruto's chakra control.
To write an 'arataka reigen naruto fanfiction', start by doing research on both series. Know the rules of the Naruto world, like chakra and jutsu. For Reigen, understand his way of talking and his motives. You could start with a simple idea, like Reigen being hired to help a ninja village with a'spiritual' problem. As you write, let the characters develop. Maybe Reigen will learn some real skills from the Naruto characters, or vice versa. Also, use vivid descriptions to bring the Naruto world to life in your story, whether it's the Hidden Leaf Village or a battle scene.
Hard to say. Maybe it will if there's enough demand and the story has more to tell. But sometimes, even popular mangas come to an end. We'll just have to wait and see.
Mob Psycho manga has indeed finished. It wrapped up with a satisfying conclusion for fans. The author completed the narrative and tied up all the loose ends.