One possible plotline could be Harry's reasons for jumping. Maybe he's under the influence of a dark spell and jumps to break free from it. Another could be that he jumps as a sacrifice to save his friends, believing it's the only way.
Voldemort could be involved too. It could be part of his plan to drive Harry to despair and jump. Or, on the other hand, Harry's jump could be a way to outwit Voldemort. For example, by faking his death in this way and then coming back stronger to defeat him.
One great Astronomy Tower fanfiction could be about a secret midnight meeting there. Maybe Harry and his friends discover a hidden passage in the tower that leads to a room filled with ancient magical artifacts. They have to keep it a secret from the teachers and the other students, especially Malfoy who they suspect might try to steal the artifacts for the wrong reasons.
Well, it depends on the context of the fanfic. If he jumps, he could be in a situation where he believes it's the only way to escape a powerful dark force on the tower. However, his Gryffindor courage might also lead to an unexpected rescue. For example, a phoenix could swoop in and carry him to safety at the last moment.
He could die. In most scenarios, jumping from such a height would be fatal, which would have a huge impact on the wizarding world. His friends like Ron and Hermione would be devastated.
A scenario could be that the Astronomy Tower is a portal to another dimension during a full moon. Only those with a pure heart can enter it. Ginny discovers this and she enters with Neville. Inside the other dimension, they find a world where magic is used in completely different ways and they have to learn new spells to get back home.
One possible reason could be to save someone else. Maybe a friend or a loved one is in danger at the base of the tower and Harry believes jumping is the quickest way to reach them. For example, if there's a dark spell about to be cast on Hermione and the only way to stop it in time is for Harry to reach her instantly.
One interesting astronomy tower fanfiction could be about an astronomer making a discovery from the astronomy tower. Maybe they spot a new comet while observing from that high vantage point. It could involve the character's excitement, the process of verifying the discovery, and how it changes their life.
I'm not sure of specific authors who only write astronomy tower fanfiction. But you could start by looking at general science - fiction or fantasy authors who might have works that involve astronomy towers. Some well - known science - fiction authors might include Isaac Asimov. He often wrote about scientific concepts and could potentially have something related in his works.
In fanfiction, if Ash jumps off Prism Tower, he could be caught by a flying - type Pokemon like Pidgeot. Pidgeot, being loyal to Ash, might sense his distress and swoop in to save him at the last moment.