If Naruto absorbs Kaguya in a fanfic, it might lead to a very different storyline. He could use Kaguya's power to bring about a new era of peace more quickly. For example, he might be able to use her power to stop all forms of aggression instantly. Also, his appearance might change slightly, taking on some of Kaguya's physical traits. And his understanding of the chakra and the world's origin could be greatly enhanced.
Well, in a fanfic where Naruto absorbs Kaguya, it's a really interesting concept. Naruto is already extremely powerful, but with Kaguya's power, he could become a sort of god - like figure. He may be able to access the different dimensions that Kaguya could control with ease. His friends and allies would probably view him with a mix of awe and fear. His newfound power could also attract new enemies who are jealous or afraid of his strength. It could also lead to Naruto having to deal with the dark side of Kaguya's power that he may not fully understand at first.