A possible fanfiction could be about the origin of the Rinne Sharingan having some connection to the world of RWBY. Maybe there was an ancient civilization in RWBY's past that had a similar power, and through some sort of dimensional rift, it influenced the development of the Rinne Sharingan in Naruto's world. This would involve a lot of world - building and exploration of both universes' histories.
A good fanfiction should have well - developed characters. It's important to show Rinne and Sakura's individual personalities before they share a kiss. This makes the moment more meaningful.
In such fanfiction, Sakura might be depicted as more alluring and perhaps more bold in her interactions with Madara. Madara could be shown as having a certain charm that draws Sakura to him.
One common theme is redemption. Naruto often uses rinne tensei to bring back characters who have made mistakes in the past and gives them a chance at redemption. Another theme is sacrifice, as using rinne tensei usually comes at a great cost in these fanfics.
Perhaps the OC's Rinne Sharingan gives them the ability to travel between different dimensions. This leads to an adventure where they accidentally bring a great evil from another dimension to their own world. They then have to use their powers in creative ways to send the evil back.
Naruto himself is often a common character. Since it's related to the Naruto universe, he might be a part of the story, either as a friend or a rival to the character with the Rinne Sharingan. Sakura could also be a common character, perhaps as one of the members of the harem.
Sasuke is often a popular character in such fanfictions. His natural Sharingan abilities make it interesting to explore how he would interact with the Rinnegan powers. Fans like to write about him achieving new levels of power by combining or evolving his Sharingan in relation to the Rinnegan.
Madara Uchiha is another popular character in Naruto Rinne Sharingan fanfiction. Since he was one of the first characters to be associated with the Rinnegan (a precursor to the Rinne Sharingan in some ways), fans are interested in seeing how his character would be even more powerful or how his motives might change if he had the Rinne Sharingan. There are often stories that revolve around his further rise to power or his redemption with the help of this powerful eye ability.
In some fanfictions, a character from another dimension enters the Naruto world with the Rinne Sharingan. This new character's power throws the balance of the Naruto world off. The story could revolve around Naruto and his friends trying to understand this new power and either make an ally of the character or defeat them to restore peace in their world.
One interesting concept could be the exploration of new jutsu combinations using the Rinne Sharingan. For example, creating unique space - time ninjutsu that surpasses what we've seen in the original series. Another might be delving into the backstory of how the Rinne Sharingan came to be in a particular fanfiction's universe, perhaps it was the result of a long - forgotten experiment.