The overall mood would likely be quite tense. Since Harry has a lot of bad memories associated with his uncle, seeing his boggart as his uncle would bring back those negative feelings. It would create an atmosphere of unease and fear every time the boggart appears in the story.
The presence of Harry's boggart as his uncle would create a mood that is both suspenseful and somewhat melancholy. Suspenseful because we don't know when the boggart will appear and what it will do. Melancholy because it represents Harry's difficult past with his uncle. The fanfiction would likely be filled with moments of Harry reflecting on his past and trying to find the strength to face this manifestation of his fear, which would contribute to a more somber and introspective mood overall.
If she looks great in her 'birthday suit' (again, assuming it's a regular, stylish outfit), it can definitely boost the mood. Her positive energy and confidence can be contagious, making others feel excited and happy to be there.
His presence could make Harry more protective. Harry might be more cautious in his adventures, not wanting to put his little brother in danger. For example, when dealing with the Marauder's Map, he might be more hesitant to use it if he thinks it could expose his brother to trouble. Also, his friends like Ron and Hermione would have to adjust to having a younger, perhaps more inexperienced, person around. They might feel responsible for him as well.
His relationships would likely be in turmoil. His friends might notice a change in Harry and be confused. Loki could make Harry act distant or say strange things to Ron and Hermione, causing them to worry. And his relationship with Ginny might also be affected as Loki might make Harry act in ways that Ginny doesn't understand.
One possible storyline could be Harry facing his fear of his uncle in a more in - depth way. Maybe he has to confront his uncle in a magical setting within the fanfiction, like in a dream - like realm where his boggart - uncle taunts him with his past memories of mistreatment. This could lead to Harry finding new ways to overcome this fear, perhaps by using a new spell he discovers.
It could potentially make Harry more empathetic towards others who are in fearful situations. Since he has experienced such intense fear himself, he may be more likely to understand and help those who are also afraid. In the fanfiction, this could be shown through his interactions with other characters who are facing their own fears.
It could boost his confidence. Knowing that the Weasleys are there watching him, Harry might feel more determined to do well.
The dragon's presence could have a significant impact on Hogwarts. For starters, it would change the security measures. The school would need to figure out how to keep the dragon safe and also protect the students from it. There might be new spells and enchantments put in place. Also, the dragon could become a part of some classes, like Care of Magical Creatures. Students would learn about its unique abilities and behaviors.
His presence could completely rewrite the history of the Hidden Leaf Village. For example, if he manages to prevent the Nine - Tails attack, it would mean that many of the characters' backstories would change. People like Sasuke, who was also affected by the events related to the Nine - Tails, might have a different path.
Harry could use his love for his friends and family. The power of love has been shown to be strong against Voldemort in the original series. So, when facing the Boggart - Voldemort, he could focus on the love he has for Hermione, Ron, and the rest of his friends, and this positive emotion could transform the Boggart into something less terrifying.
His presence could strengthen their relationship. If Harry were to save them from a dangerous situation, it might make them bond more over the shared experience. Or he could give them some advice that helps them understand each other better.