His abilities would change in multiple ways. Firstly, his chakra control might be affected. Since Godzilla is a massive and powerful entity, it could disrupt Naruto's normal chakra flow. However, he could also learn to harness Godzilla's power within him. For example, he might be able to use Godzilla's raw strength to enhance his Sage Mode or even his Nine - Tails transformation. This could make him an even more formidable opponent. His speed could also be enhanced as Godzilla's power might give him a boost in his physical capabilities, allowing him to move faster and react quicker in battle.
Well, it could make for a really interesting story. Naruto having greed sealed in him might cause him to make some uncharacteristic decisions. Maybe he'd start being more competitive in a negative way, like trying to outdo Sasuke not just for the sake of friendly rivalry but out of pure greed for power. His interactions with his mentors like Kakashi would also change. Kakashi would probably notice this new side of Naruto and try to help him overcome it.
I'm not entirely sure if there is a specific fanfiction with exactly that premise. However, given the popularity of both Naruto and the concept of sealing powerful entities in Naruto's world, it's quite possible. Fanfiction writers love to experiment with combining different fictional elements, so it wouldn't be surprising if someone has written a story where Jenova is sealed in Naruto. You can try searching on popular fanfiction websites like Fanfiction.net or Archive of Our Own to find out for sure.
In many such fanfictions, it could be a variety of deities. It might be an ancient and powerful god from Naruto's world's mythology that the story's author has chosen to insert. For example, it could be a god of elements like a super - powerful fire god, which could give Naruto even more control over his existing fire - based jutsu and new, more extreme abilities related to fire.
His character would likely become more complex. He might start off confused by these new feelings of greed. At first, he could try to resist them, but as time goes on, they might start to seep into his actions. For instance, when it comes to training, instead of training to get stronger for the sake of others, he might train just to be the best and have more power than anyone else.
With his enemies, it would be a whole different story. They might be more cautious about attacking Naruto. Some might even try to find a way to use the god sealed in him for their own purposes. And Naruto's relationship with the hidden villages could also change. They might see him as a potential threat or a very valuable asset.
Well, Naruto might become more mysterious. Since Gaster is an unknown and rather enigmatic entity, Naruto could start having strange thoughts or powers that he doesn't fully understand at first. He might seem more withdrawn as he tries to figure out what's going on inside him, and his normal cheerful and straightforward personality could be clouded by the presence of Gaster.
In such a fanfiction, Naruto might gain some extraordinary powers related to the phoenix. For example, he could have the ability to heal quickly like the phoenix's regenerative powers. He may also be able to use fire - based attacks that are far more powerful and unique compared to his regular jutsu.
Well, one possibility could be that Naruto gains new powers related to the Nibi. He might be able to use some of its unique chakra abilities, like enhanced speed or stealth. Maybe he could also communicate with other Nibi - related things in the Naruto world.
In some fanfictions, the main antagonist could be Naruto's own inner demons brought on by the presence of the wolf. These could manifest as self - doubt, fear, or a dark side of his personality that the wolf's power seems to amplify. So, in a way, Naruto has to fight against himself as well as any external threats.