Well, one interesting idea could be a crossover of Naruto's ninja world with the Twilight Saga's supernatural elements. For example, Naruto characters having to deal with vampires and werewolves. Maybe Naruto himself could learn some unique abilities from the Twilight characters.
Naruto himself is often very popular as he is the main character of the original Naruto series. Fans like to see how he would interact with Saiyan elements. Goku from the Saiyan saga is also a popular character to include. His powerful Saiyan abilities make for interesting crossovers with Naruto's world.
One interesting concept could be the fusion of Naruto's ninja abilities with Saiyan powers. For example, Naruto might use his chakra in new ways inspired by Saiyan energy control. Another concept could be the interaction between Naruto's friends and Saiyan characters. They might learn from each other's fighting styles and cultures.
Well, you could write a fanfiction where Tanya, with her knowledge of modern warfare and magic, becomes a mentor to some of the younger ninjas in Naruto. She could teach them new ways of strategizing and fighting. Also, a plot where Tanya gets involved in the Akatsuki's plans, either trying to stop them or using them for her own gain. And there's the possibility of a story where Tanya's unique abilities are mistaken for a new and powerful ninja jutsu, causing all sorts of chaos and intrigue in the Naruto world.
In a Naruto Saga fanfiction, you could explore a plot where Naruto is sent to an alternate dimension where the rules of chakra and ninja abilities are different. He has to adapt quickly and find a way to return home while also learning about this new world and its secrets. This could lead to him developing new skills and relationships that he can bring back to his own world.