One common theme is self - discovery. When Naruto regresses in age, he often has to re - learn things about himself and his abilities. For example, he might rediscover his ninja skills in a different way. Another theme is friendship. His relationships with his friends are re - examined, like how Sasuke might react to a younger Naruto.
Change is a very important theme. The entire Naruto world is different when he regresses. His actions and decisions have different consequences. This can range from small things like how he approaches his training, to big things like how he might affect the balance of power in the ninja world. It's all about exploring how one change in Naruto's state can have a ripple effect on the whole story.