One possible consequence could be chaos in the ninja world. With Naruto, a major force for peace, dead and the Kyuubi released, other villages might see an opportunity to gain power or territory. Villages that once relied on Naruto's protection would be vulnerable. Also, the Kyuubi, being a powerful and wild force, could start causing destruction on its own, without Naruto to control it.
One possible reason could be that the Kyuubi's power goes out of control completely. Since Naruto has the Kyuubi inside him, if he fails to maintain the balance, the Kyuubi's raw and unrestrained power might end up killing him.
Well, in fanfiction, the Naruto and Kyuubi fusion could lead to a change in how the ninja world views Naruto. He would be seen as an even greater threat or an even more powerful ally. For example, other villages might be more hesitant to engage in conflict with him. Also, his relationship with his friends could be affected. They might be in awe of his new power but also a bit worried about the Kyuubi's influence on him. And on a physical level, his appearance might change, perhaps taking on some of the Kyuubi's features, like his eyes or his hair color getting a more reddish tint.
One possible consequence is that the power balance in the ninja world is completely disrupted. With the Kyuubi's massive chakra and power running free, smaller villages might be constantly in fear of being attacked. Konoha, in particular, would have to find new ways to defend itself without relying on the fact that the Kyuubi is sealed within one of its shinobi.
Well, his relationship with the village could be affected. The villagers who were once afraid of the Kyuubi might be even more hesitant to accept Naruto now that he has the complete power. However, his closer friends like Sasuke and Sakura would stand by him. Sasuke might see Naruto as an even greater rival, but also a potential ally in protecting the village. And Sakura would be there to support Naruto both emotionally and in battle if needed.
Madara Uchiha could also be a main antagonist in this fanfiction. He has grand plans for the ninja world and the released Kyuubi could be a key part of his scheme. Since Naruto is dead and can't oppose him, he might try to use the Kyuubi to reshape the world according to his vision. He has the power and the ambition to be a major threat in this situation.
One consequence could be that he becomes extremely powerful. His physical abilities like strength, speed, and endurance are enhanced. He can also use more powerful jutsus with the extra chakra boost from the Kyuubi.
One consequence could be a huge power boost. Naruto would become extremely strong, which might change the power balance in the fanfiction world. Enemies would be more cautious of him, and allies might rely on him more. Another consequence could be the internal struggle. The Kyuubi has its own will, so Naruto might have to constantly fight to maintain control when it's summoned.
If the fem Kyuubi gets control of Naruto in the fanfiction, one consequence could be that Naruto's relationships with his friends might be strained. His friends might not trust him anymore as they notice the change in his behavior under the Kyuubi's control. Another possibility is that Naruto could be used to cause chaos in the village. Since he's very powerful, the fem Kyuubi could use him to carry out her own plans, which might be harmful to the village and its people.
One possible consequence is that Naruto might lose control of his own actions. The Kyuubi is a powerful and often uncontrollable force. So, he could end up causing a great deal of destruction without meaning to. For example, he might attack his friends or allies in the heat of the moment as the Kyuubi's power overwhelms his own judgment.
In some fanfictions, Naruto's mother, Kushina, has a special connection with the Kyuubi. For example, some stories might explore how Kushina's chakra influenced the Kyuubi's behavior. Since she was the Jinchuriki before Naruto, her relationship with the Kyuubi could be a rich source of plot development in fanfiction.