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.
One possibility is that Naruto's chakra nature could change completely. Since Kaguya had unique chakra, Naruto might develop new jutsu that are a blend of his own and Kaguya's abilities. For example, his Rasengan could gain the power to not only attack but also to heal, as Kaguya had some form of life - giving power. Also, his connection to the tailed beasts could be enhanced in unexpected ways, allowing for better communication and cooperation with them.
Naruto's skin might start to glow with a strange energy, similar to how Kaguya seemed to have an otherworldly aura. His eyes could change color to something more like Kaguya's, perhaps a deeper, more mysterious hue. And his clothing might even transform to be more in line with Kaguya's style, as if the absorption of her power also influenced his physical form in that way. He might also grow taller or more muscular as a result of the new power coursing through his body.
Well, if Naruto absorbs Kaguya in a crossover fanfiction, it could drastically change the power balance. Naruto already has a lot of power, and adding Kaguya's abilities would make him nearly invincible. It might also change his relationships. His friends could be in awe or a bit scared of his newfound power. And it would surely change the way he faces future enemies. Instead of just fighting, he could potentially just overpower them with his new combined abilities.
Well, one possibility is that Naruto could use his newfound power of absorbing Kaguya along with the time travel ability to prevent some of the major tragedies in the Naruto universe. For example, he might stop the Uchiha massacre, which would change the entire course of the story. It could also lead to a different relationship between Sasuke and Naruto, as Sasuke's motivation for revenge would be gone.
His sense of responsibility would likely increase exponentially. Since he now has the power to potentially change the past and shape the future, he would feel a great burden. This could lead to him becoming more of a strategic thinker, rather than just relying on his brute strength and determination. He would also have to deal with the moral implications of using his new powers, which would further shape his character.
Kakashi can be a popular character too. His wisdom and combat skills can be useful in fanfic scenarios involving Kaguya. He could be a mentor figure to Naruto, guiding him in dealing with Kaguya - related threats or helping to decipher ancient ninja knowledge related to Kaguya.
Well, it could completely change the course of the story. For example, if Naruto travels back in time, he might be able to prevent certain events that led to Kaguya's power becoming so dominant. He could also use his knowledge of future events to train himself or others more effectively, thus altering the power balance in the ninja world much earlier.
If Naruto absorbs the Juubi in fanfic, it could change the whole Naruto universe in that story. He may have new forms or abilities. Maybe he could use the Juubi's power to bring peace to the ninja world more quickly. He could also face new challenges as other characters might fear his new - found power.
One common theme is redemption. Since Kaguya was initially a villain, many fanfics explore her journey towards redemption with Naruto's help. Another theme is power. How their combined powers can be used for different purposes, like protecting the ninja world. For example, in some fanfics, they use their powers to fight off a new threat that is stronger than any they've faced before.