How about a story where the Monster Musume and Naruto characters accidentally swap bodies? This would lead to a lot of humorous situations as they try to adapt to each other's lives. They would have to figure out how to use their new bodies' abilities and also deal with the reactions of their friends and family. As they struggle to get back to their original bodies, they would learn more about each other's worlds and cultures. This could also be a chance for some of the characters to gain new perspectives and change for the better.
One idea could be a story where Naruto and Haku team up on a special mission in a new land. They have to face a powerful enemy that uses unique jutsu. Haku can use his ice skills in creative ways to help Naruto, and Naruto can show his growth in terms of his Rasengan and new sage mode abilities.
One idea could be that Naruto discovers a hidden village of monster girls in the Land of Fire. He has to learn to co - exist with them and maybe even team up with some to face a new threat that targets both humans and monster musume.
One idea could be a time - travel fanfic where a character goes back in time to prevent a major disaster in the Naruto world. This would involve a lot of strategic planning and interaction with different characters from different time periods.
A possible plot could be that Female Kakashi discovers a new and dangerous jutsu that only Naruto can help her control. They embark on a journey to master it together, facing various challenges along the way. This could involve them having to battle rogue ninjas who want to steal the jutsu for themselves.
Another idea is that Zelda characters find themselves in the Naruto world. They could get involved in the ninja academy and learn about chakra. Maybe Zelda herself could try to master some of the ninja arts and use her magic in combination with it. This would lead to a lot of interesting interactions between the Zelda characters and the ninja students.
One plot idea could be a training arc. Naruto could teach Illya some basic ninja skills like stealth or tree - climbing, while Illya could show Naruto some of her magic spells for defense. Another idea is a mystery - solving plot where they team up to figure out why strange things are happening in both their worlds. For example, strange creatures are appearing in Konoha and in Illya's town that seem to be related.
A possible plot is that the main character discovers a hidden power within himself that attracts all the monster girls. As the harem forms, they have to deal with the outside world's reactions. The 'double' part could mean there are two main groups of monster girls, perhaps from different regions or species. Regarding the 'lemon' element, it could be about the characters exploring their new - found feelings through soft and passionate embraces. For instance, a cat - girl might purr contentedly when hugged by the main character, leading to a deeper emotional connection.
One idea could be a story where a young Bokoblin gets separated from its pack and is found by Zelda. Instead of harming it, Zelda tries to teach it kindness and cooperation. The Bokoblin could then become an ally to Zelda, helping her understand the ways of the Bokoblins better, and they could work together to solve some problems in the Zelda universe, like finding a way to stop a natural disaster threatening Hyrule.
One plot idea could be a female Sasuke who uses her unique skills in a different way than the male Sasuke in the original. For example, she might focus more on diplomatic solutions to problems in the ninja world, using her intelligence and combat skills to broker peace treaties.
Another concept might be the idea of forming teams or partnerships. Just as in Naruto where characters form teams for missions, in a 'naruto and monster musume fanfic', a ninja could team up with a monster musume. They could combine their abilities, like a monster musume with enhanced strength working together with a ninja who has stealth and jutsu skills to complete a task that neither could do alone.