One possible consequence is that the demigod world would be thrown into disarray. Percy has been a key figure in maintaining some sort of order, and with Kronos in control, that order would be shattered. Another consequence could be that the Gods would be in a real bind. They already had trouble with Kronos before, and now with him in Percy's body, it would be much harder to fight him.
Well, one possible consequence could be that Percy gains an immense amount of power. Kronos was a very powerful Titan, so Percy might become nearly invincible. He could also face some internal struggles as Kronos' essence might try to influence his mind and actions.
One possible consequence is that Naruto's body might not be able to handle Kurama's full power all at once. It could lead to physical harm or even death if not managed properly. Also, his relationships with Sakura, Sasuke and others would be in jeopardy as they might not recognize the Naruto under Kurama's control.
On a personal level, Percy might have to deal with the emotional toll of killing Kronos. It could be a very traumatic experience for him, even if Kronos was an enemy. He might have nightmares or second - guess his actions. Also, his relationships with other characters might change. Some might fear his new - found power, while others might be more drawn to him because of his victory.
One consequence could be Percy becoming an outcast among his fellow demigods. Since his powers now come from Kronos, they might see him as a threat or a corrupted being.
Well, Kronos is a very powerful and evil figure. If he takes over Percy in fanfiction, Percy might be forced to do a lot of bad things against his will. His friends would likely be in great danger as Percy, under Kronos' control, could turn on them. Kronos could use Percy's powers to cause chaos and destruction in the world of the Gods and demigods.
In fanfiction, Percy could become a champion of Kronos due to a complex web of factors. It could start with his disillusionment with the Olympians. After all the battles he's fought for them, if he sees no real change in how they treat demigods or the world in general, he might be open to an alternative. Kronos, being a master manipulator in these alternate stories, could offer Percy power and a chance to truly make a difference. Also, Percy's relationships could play a role. If someone he trusts sides with Kronos and convinces him, or if he's in a situation where he has no other allies and Kronos offers support, he could end up in that role. But these are all departures from his original character in the official Percy Jackson series.
Adrien might find himself being influenced by the power of the brooch in unexpected ways. The brooch has a connection to the akumas and their creation, so it could potentially corrupt him or make him act out of character. He might also be ostracized by his friends if they discover he has the brooch, as they might not understand his motives.
In the fanfiction, Kronos may be presented as having a more complex personality. His actions that seemed evil in the original story could be a means to an end. For example, he might have been trying to save the Titan race from extinction at the hands of the gods, which is a valid reason and thus makes him not purely evil in the context of the fanfiction.
Well, perhaps in the fanfiction, Percy is tortured by Kronos in a way that involves his demigod powers being slowly drained. This would make him feel weak and vulnerable, which is a huge contrast to his usual strong self. His torture could also be a way for Kronos to try and break his spirit so that he can turn Percy to his side. Meanwhile, the gods might be debating whether or not to interfere, adding another layer of conflict to the story.
In fanfiction, when Percy meets Kronos in Tartarus, Percy might be initially terrified. But he soon gathers his courage. Kronos might start taunting Percy, saying things like how he is a fool to come to Tartarus. Percy, in response, challenges Kronos' view of power and domination. They then start a battle of ideologies as much as a physical battle, with Percy arguing for the good of the world and Kronos trying to justify his past actions of trying to take over.