In general, the Twin Star Exorcist anime remains loyal to the manga. It manages to capture the key elements and the overall tone of the story. Some changes are made for better animation presentation and to control the pacing, but they are not substantial and don't stray far from the manga's core.
Overall, the Twin Star Exorcist show has some similarities to the manga, but there are also added or changed elements to make it more suitable for the screen. Some fans think it's a decent adaptation, while others have mixed feelings.
A possible fanfiction could involve a character from Naruto, say Sasuke, ending up in the Twin Star Exorcist world. Sasuke, with his Sharingan and powerful jutsu, could be mistaken for an exorcist at first. He then has to figure out how to get back to his own world while also dealing with the Kegare. Or perhaps Naruto and Rokuro could meet in a strange in - between world. They could team up to fight a common enemy that is a threat to both their worlds.