Well, perhaps they begin by magically cleaning the flat first. After that, they might decide to put up some pictures on the walls. Sirius could bring out some old photos of Harry's parents that he's been saving. They also might use some spells to create floating candles for a cool lighting effect. And they could make a cozy corner with a big, plush armchair and a bookshelf filled with Harry's favorite books.
They might start by choosing a color scheme. Maybe they'd go for Gryffindor colors like red and gold to show Harry's house pride. Sirius might use his magic to move the furniture around easily while Harry decides where to put his Quidditch memorabilia. They could also add some protective enchantments around the flat while they decorate, making it a safe and cozy place.
I think Sirius would be the one. He's been in and out of different places, from his family's old manor to Azkaban. He has a broader view of what's possible. He could suggest things like using a time - turner as a decorative piece (in a non - functional way) or creating a ceiling that mimics the night sky at Hogwarts, which are very creative ideas compared to what Harry might come up with.
Sirius has a wild side. He could suggest having a room dedicated to pranks, filled with things like extendable ears and dungbombs (just for fun, not for actual use). Harry, being responsible, would be more concerned about the security of the flat. He'd make sure the doors and windows have strong locking spells. He might also want a study area that is quiet and organized for his studies and future plans.
Well, it could be about Harry suddenly finding out he has some sort of lordship that was previously unknown. Maybe it's related to an old wizarding family. He might have to deal with new responsibilities, like managing estates or dealing with family secrets.
One reason could be Harry's impulsiveness. Harry often jumps into dangerous situations without thinking, and Sirius, being his godfather and having a protective nature, would shout at him to make him realize the risks. For instance, if Harry decides to go after a Death Eater alone.
Perhaps there are dark wizards who don't want Harry to gain more power through this lordship. They might fear that he'll use it to oppose them more effectively. So they would plot to stop him from fully taking on his new role.
Sirius could help Harry cope by sharing his own experiences of being an outcast in his family. He could tell Harry that just because his parents did this horrible thing, it doesn't mean he's unlovable. Sirius could also introduce Harry to his friends in the wizarding world, like Remus, who could also offer support and friendship.
It's possible that in the cave, they start to see each other as equals rather than just a mentor - student relationship. They might argue about the best way to deal with the cave's mysteries at first, but through that process, they learn to value each other's perspectives. Also, if one of them gets injured in the cave, the care and concern shown by the other would enhance their relationship.
Yes, there might be such fanfictions out there. Fanfiction writers often come up with creative and sometimes out - of - the - box ideas. They might explore the idea of Harry having a different relationship to Sirius in an alternate universe where he could be seen as Sirius's daughter. However, it's not a common or traditional take on the Harry Potter universe.