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 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.
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.
Ron would also be important. His loyalty to Harry is unwavering. He might be the one who bravely volunteers to go on the most dangerous part of the rescue mission, perhaps facing off against some of Voldemort's most feared Death Eaters.
Hermione could be one. She might use her vast knowledge of magic to help Harry figure out how to hatch the egg. Another could be Hagrid. He has a lot of experience with magical creatures, so he could offer advice on taking care of the egg and the soon - to - be - hatched creature.
Hermione and Ron would also be important characters. Hermione might be initially skeptical of Harry's newfound obsession with the Marauders. Ron, on the other hand, might be more easily swayed and join Harry in his attempts to be more like the Marauders.
A vampire hunter could be a key character. They might initially see Harry as a target too because of his association with the vampire. This would create a lot of tension as Harry and his mate have to either convince the hunter that they are not a threat or fight against them. And don't forget about Dumbledore (assuming this is in the Harry Potter universe). His reaction to Harry's new situation would be very interesting to explore.
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.
There could be his drill sergeant. A tough but fair character who pushes Harry to his limits. Maybe he has a name like Sergeant Thompson.
Well, in some cases, there could be minor characters from the original series who still interact with Harry on a limited basis. For example, Neville Longbottom might make an appearance. Even though Harry is alone for the most part, Neville could cross paths with him and have a brief conversation or exchange that has an impact on Harry's journey.
There could be Ron. He might be Harry's sidekick in the gambling adventures, either joining him in bets or trying to stop him when things get out of hand. Ron's loyalty would play a big part in their relationship within the story.