You could write a fanfiction where the Power Rangers Ninja Storm team has to train a new generation of Rangers. This would involve them passing on their knowledge and skills. It could also explore how they adapt to being mentors. Another idea is to set the story in a future time where the original Rangers are long gone, but their legacy inspires a new team to take on the mantle. There could be a mystery element where the new team has to discover the secrets of the Ninja Storm powers that have been lost over time.
A cool fanfiction concept could be to have the Ninja Storm Rangers lose their powers temporarily and have to rely on their martial arts skills alone to fight the bad guys. This would show their growth as individuals and their bond as a team. Or, you could write about an alternate universe where the Rangers have different personalities but still have to come together to save the world. You could also focus on a single Ranger and their internal struggles, like Shane dealing with the pressure of being the Red Ranger.
One idea could be exploring the backstories of the Ninja Storm Rangers in more depth. For example, what were their lives like before becoming Rangers? Another could be a crossover with another Power Rangers series, like how they would interact with the Dino Thunder Rangers. Maybe a story where they face a new, more powerful villain that forces them to develop new powers or strategies.