It might start with curiosity. Naruto, coming from his own time, is intrigued by Naruko. They then start training together. As they train, they see each other's strengths and weaknesses, which brings them closer.
One interesting plot could be Naruto time - traveling back to his childhood and meeting a female version of himself, Naruko. They could team up to change the future of the Hidden Leaf Village. Maybe they work together to prevent certain tragedies that happened in the original timeline.
One common theme is redemption. They often go back in time to correct past mistakes, whether it was Naruto's lonely childhood or some major events that went wrong in the original storyline. For example, in many fanfictions, they try to save characters who died in the original plot.
Another theme is power - up. Since they have knowledge from the future, they can train more effectively or use future - knowledge - based strategies to become stronger. For instance, Naruto could use his knowledge of future battles to develop new jutsus faster. It's also about changing destiny. They don't want to accept the future as it was in their original timeline and strive to create a better one for themselves and their village.
Danzo Shimura might be another one. He is very secretive and power - hungry. If Naruto and Naruko's time - traveling actions expose his dark deeds or interfere with his control over the village, he would become their enemy. He has many underhanded methods and a strong influence within the village that they would have to deal with.
One popular 'naruto x naruko time travel fanfiction' is 'Naruto: Time Traveler's Redemption'. In this story, Naruto and Naruko are sent back in time and have to change the course of events to prevent a great tragedy. It has an interesting plot that delves deep into the characters' emotions and how they deal with the knowledge of the future.
For Naruto, you can develop his character by having him face new challenges related to his relationship with Narumi and Naruko. For example, he might struggle with how to balance his friendship with both of them. Narumi could be developed by giving her a backstory that ties into the Naruto universe in a unique way, like she has a family connection to an old ninja clan. And for Naruko, you can explore her more feminine side in contrast to Naruto's more brash nature, especially when it comes to her interactions with Narumi.
For Naruto, you could show his growth in terms of his leadership skills when he's with Naruko and Kushina. He could learn to be more patient and understanding, especially when Naruko might have a different approach to things. For Naruko, her character could be developed by showing her struggle to be recognized in a world where Naruto already has a reputation. Kushina could be developed by exploring her motherly side more deeply, as she interacts with both Naruto and Naruko.
One possible fanfiction could be about Naruko traveling to different dimensions in the Naruto universe and meeting alternate versions of her friends and enemies. She might learn new jutsus or gain unique powers from these encounters.
There's also 'Naruko's Time - Travel Adventure'. In this fanfic, Naruko goes back in time alone and has to face many challenges. She uses her knowledge from the future to deal with enemies and also tries to save some of the characters who died in the original timeline. It's a really exciting read with lots of character development.