Obviously Naruto and Sonic are common characters as it's in the title. There could also be female characters like Sakura from Naruto series and Amy Rose from Sonic series.
One interesting plot could be that Naruto and Sonic find themselves in a strange new world where they have to protect a group of female characters from a common enemy. As they do this, they form deep bonds with these characters, leading to a harem - like situation.
One common theme is adventure. Sonic is often on some sort of journey or mission, and his harem members play different roles in it. For example, they might help him fight enemies or solve puzzles.
One great Sonic X harem fanfiction is 'Sonic's Multiverse Harem'. It has an interesting plot where Sonic from different dimensions meets various characters and forms a sort of harem relationship. The story is full of action and adventure along with the harem elements.
Sakura could be in the harem. She has a long - standing relationship with Naruto in the original series, and in this fanfiction, his Saiyan powers might make her see him in a new light. Hinata is also a likely candidate. Her admiration for Naruto could turn into a deeper relationship given his Saiyan - like abilities.
One common theme is friendship. Since both Naruto and Sonic are heroic characters, stories often focus on how they become friends and learn from each other. For example, Naruto's never - give - up attitude might inspire Sonic.
Sakura could be one of the members. She has always had a complex relationship with Naruto. In this harem scenario, her feelings could evolve further. Hinata is also a likely candidate. Her long - standing admiration for Naruto might lead her to be part of the harem. Tenten could also join, perhaps drawn in by Naruto's growth as a ninja.
Well, obviously Rias herself would be a central member. Then there could be Akeno Himejima. She has a mysterious and alluring personality, and her relationship with Rias could be an interesting dynamic when Naruto enters the picture. Also, Koneko Toujou, with her cute and somewhat tsundere nature, might be another member. She could be initially cold to Naruto but gradually warm up to him.
In the Naruto x harem fanfiction, Naruto is a badass because of his combination of raw strength, strategic thinking, and his heart of a hero. He's not afraid to go against the odds and always finds a way to come out on top for himself and his harem.
One interesting crossover could be the characters' different abilities interacting. For example, Naruto's shadow clones and Sonic's speed could be combined in a unique way during a battle. Maybe they team up to fight a common enemy that threatens both their worlds.