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.
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.
They might be really surprised. Some characters could be amused by the creative scenarios fans come up with. For example, Hinata might be excited to see himself in all kinds of wild adventures in the fanfictions.
I have no definite answer. Sometimes manga series end when the story feels complete to the creator or when publishing circumstances change. So, we'll just have to wait and see.