Kageyama stays with Karasuno in the manga. His character development and contributions to the team continue without him leaving. The bond he forms with his teammates keeps him committed to the club.
Sure, Karasuno beats Shiratorizawa in the manga. The match is full of thrilling moments and showcases the growth and determination of the Karasuno team.
He would probably be quite amused by some of his past antics. For example, his constant pranks on Iruka-sensei. But at the same time, he'd also see the pain he carried from not having his parents and realize how that influenced his actions. He might react with a bit of regret for some of the stupid things he did out of loneliness, but also be glad that those experiences made him who he is today.
One possible storyline could be a friendly exhibition match. The Karasuno boys are initially overconfident, thinking they'll easily win against the girl team. But as the match progresses, they realize the girls are much more skilled and tenacious than they expected. Another storyline could involve the girl team challenging Karasuno to a match to prove that girls can be just as good at volleyball. And there could be a sub - plot of a Karasuno player falling for a girl on the opposing team, adding some romantic tension to the story.
In a fanfic, it could really go either way. The Karasuno team has strong male players with different skills. But the girl team might have their own unique strategies and teamwork. If the Karasuno boys rely on their power and height, like Asahi's powerful spikes, they could have an advantage. However, the girl team could use their agility and quick thinking to outmaneuver them. It would make for an exciting match in the fanfic.
Well, some past gods might be amused. They'd see these modern interpretations of their stories and characters in the Percy Jackson fanfiction as a strange yet interesting new form of worship or at least attention. For example, Zeus might be a bit irked at some of the liberties taken with his character, but also secretly flattered that people are still so interested in him.
Captain America would probably be very reflective. He might see his past self as a reminder of his long journey and how much he has changed. He could try to offer some words of wisdom to his younger self.