It could change the plot drastically. Q has the power to manipulate reality, so he could change the fates of characters in the Harry Potter world. For example, he might save some characters who were supposed to die or create new challenges for Harry that are not part of the original story.
The encounter would likely introduce a lot of chaos. Q's unpredictable nature means he could disrupt the carefully crafted plot of Harry Potter. He might expose secrets that were meant to be hidden, or he could give Harry knowledge that he wasn't supposed to have yet. This would force Harry to make different decisions and could lead to a completely different path for the story. For instance, if Q reveals the true identity of a character early on, it could change the way Harry interacts with that person and the events that follow.
The interaction could be full of surprises. Q might be amused by Harry's magic, which is so different from his own powers. Harry, on the other hand, would be in awe of Q's abilities. Q could communicate with Harry in a rather cryptic way at first, making Harry work hard to understand what he means.
If Loki were to enter the Harry Potter world, it could significantly disrupt the plot. His ability to deceive and manipulate could lead to many characters making wrong decisions. For instance, he could trick Dumbledore into believing a false prophecy, which would send Harry and his friends on a wild goose chase.
The encounter might start with a sense of mystery. Harry could be drawn to a hidden part of Hogwarts where he stumbles upon Slytherin. Slytherin would likely be guarded, not knowing who this strange boy is. The author could show the encounter through the use of vivid imagery, like the dimly lit chamber where they meet. Their conversation could revolve around the future of Hogwarts and how Slytherin's ideals have been somewhat misinterpreted over the years.
In a fanfic, the encounter between Harry Potter and Death could be shown in a more philosophical way. Death could be portrayed as a wise being, not menacing but rather calm. Harry and Death might sit down, like in a dream - like landscape, and have a long conversation about life, death, love, and sacrifice. The scene could be filled with soft light and a gentle breeze, symbolizing the peaceful nature of the discussion despite its heavy topic.
His anger could drive him to take risks he wouldn't normally take. Maybe he decides to go after a powerful enemy on his own without proper backup because he's so angry that he doesn't think clearly. This could put him in great danger and also change the course of the story as the other characters then have to scramble to save him.
They could also have a connection to his emotions. In fanfiction, perhaps when Harry is feeling angry, the tattoos might darken or become more prominent. When he is happy, they could shine brightly. This connection between his emotions and the tattoos could be a way for him to better understand himself and also for others around him to gauge his mood without him having to say a word. And the tattoos could potentially attract certain magical energies based on his emotional state, further affecting his magic.
It would likely shock and outrage many fans. The Harry Potter fan community values the original characters and storylines. This kind of fanfiction goes against the core values of the community.
Some of Harry's friends might be initially shocked. Ron, for instance, could be quite skeptical and voice his concerns about the age difference. Hermione, on the other hand, might be more open - minded at first, but as she sees how the relationship impacts Harry's studies or his role in the fight against dark forces, she could start to have doubts.
Loki's mischievous personality could make Harry more adventurous. Harry might start to take more risks in his magical studies and in facing Voldemort. For example, he could be more willing to try out untested spells or strategies.
Kreacher is a very traditional character. His influence on Harry could mean that Harry is more into the old magic. He might be taught spells and incantations that are not commonly known in modern wizarding society. Also, Kreacher's loyalty to the Black family could make Harry more protective of the remaining members of the family and their heritage.