Elizabeth from Nanatsu no Taizai and Rias from High School DxD would make for an interesting duo. Elizabeth is gentle and kind, while Rias is a strong and confident devil. They could have a great dynamic, with Elizabeth's compassion influencing Rias's more calculating nature, and vice versa. Another interesting combination would be Gowther from Nanatsu no Taizai and Akeno from High School DxD. Gowther's mysterious and emotion - analyzing abilities combined with Akeno's powerful and alluring personality would be quite a sight to behold.
One of the most interesting combinations could be Escanor from Nanatsu no Taizai and Xenovia from High School DxD. Escanor's power that varies with the time of day and Xenovia's strong will and combat skills would be an amazing match. They could learn a great deal from each other in terms of fighting style and mental strength. Also, Merlin from Nanatsu no Taizai and Gasper from High School DxD would be a curious combination. Merlin's vast magic knowledge and Gasper's unique vampire abilities could lead to some very strange and interesting adventures together.
Meliodas and Issei would be a very interesting combination. Meliodas is a powerful and somewhat mischievous character from Nanatsu no Taizai, while Issei is the perverted but determined protagonist of High School DxD. Their personalities are so different that it would create a lot of comical and also intense situations. For example, Issei might be constantly trying to learn Meliodas's techniques for getting girls, while Meliodas could be teaching him some valuable combat skills in return.
One interesting plot idea could be the characters from Nanatsu no Taizai and High School DxD teaming up to fight a common enemy that threatens both their worlds. Maybe a powerful demon from the DxD universe that has found a way to cross over to the world of Nanatsu no Taizai and is causing chaos there. Another idea is a story about the exchange of powers. For example, a character from Nanatsu no Taizai learning some of the Sacred Gear techniques from High School DxD and vice versa.
One possible plot could be the characters from both series attending a joint high - school event. For example, the Seven Deadly Sins characters could participate in a sports tournament at the High School DxD's school. Meliodas could show off his incredible strength in the athletic events, while Issei from High School DxD might be trying to impress the girls with his newfound powers. This would lead to some friendly rivalries and also some unexpected alliances as they face off against other schools in the tournament.
It's hard to say for sure. Naruto has his Sage Mode and Kurama's power, which makes him extremely strong. But Meliodas from The Seven Deadly Sins has his demon powers and is also a very skilled fighter. I think it would depend on the situation and how the writer wants to portray their abilities.
Meliodas is always an interesting character to focus on. His complex past, his relationship with Elizabeth, and his role as the captain of the Seven Deadly Sins make him a central figure. In a fanfiction about 'perfect time', his immortality could play an interesting role. He might have a different perspective on time compared to the others.
Meliodas is a very suitable character for crossovers. His powerful abilities and complex personality can bring a lot to the story. He could interact with other powerful characters from different series and either form alliances or rivalries. For example, if crossed over with a character like Naruto, his determination and fighting skills could be compared and contrasted.
Ichigo and Issei would be a very interesting pairing. Ichigo is a powerful Soul Reaper with a strong sense of justice, while Issei is a perverted but determined devil. Their personalities are so different, yet they both have a drive to protect. They could learn a great deal from each other. Ichigo could teach Issei about self - control and discipline, while Issei could introduce Ichigo to a more carefree way of looking at things.
Jiraiya would also be a great character in this fanfiction. His pervy yet wise nature combined with his knowledge of senjutsu could lead to some really funny and also some very deep moments in the story. He could interact with the characters in High School DxD in a unique way, perhaps teaching them some of his own skills or learning from them as well.
I think Sasuke and Issei could make for a great combination. Issei is all about power and his perverted nature adds a comical element. Sasuke, on the other hand, is serious and highly skilled. Their dynamic could be really fun to explore. Issei might be in awe of Sasuke's skills and try to learn from him, while Sasuke could be both amused and irritated by Issei's antics.
Gohan and Issei would be really interesting. Issei is all about being a pervert and getting stronger in his own way. Gohan is this powerful yet humble Saiyan. Their different outlooks on power and relationships would create a lot of humorous and thought - provoking moments.