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.
It could be about Sirius teaching Harry advanced magic skills like Patronus charm. Sirius, being a skilled wizard, might train Harry to be better at dueling and facing dark wizards.
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.
One possible reason could be Sirius believing he can protect Harry better on his own. Maybe he thinks the Dursleys are too dangerous for Harry and that the wizarding world isn't doing enough. So, he decides to take matters into his own hands and kidnap Harry to keep him safe.
Perhaps there could be a plot where Sirius and Harry team up to uncover secrets about the Dark Arts that are hidden in the Black family home. As they explore the old rooms and artifacts, they might stumble upon a powerful and dangerous object that the Death Eaters are also after. This would lead to a race against time to either destroy or protect the object while also fending off the Death Eaters.
Well, perhaps Sirius trains Harry in advanced defensive magic. Harry could learn spells like the Patronus charm more effectively under Sirius's tutelage. Sirius might also teach Harry about the wizarding world's secrets that Dumbledore didn't share.
One possible plot could be that Harry and Sirius' daughter grow up together. They might start as friends, sharing adventures at Hogwarts. As they get older, they realize their feelings for each other are more than just friendship. Harry, being the hero he is, would be very protective of her. Their relationship could face challenges like dealing with the memories of Sirius and the expectations of the wizarding world. Maybe they have to fight off some dark forces together which makes their bond even stronger, leading to their marriage.
One possible plot could be that Harry discovers the truth about her real parentage early on. She then has to deal with the implications of being Sirius' daughter while still at Hogwarts, facing both the admiration and suspicion from others.
One possible plot could be that Sirius accidentally casts a spell that turns Harry into a baby. He then has to take care of Harry in this new form while trying to find a way to reverse the spell. He might face challenges like hiding Harry from the wrong people, and also dealing with Harry's baby - like needs such as feeding and changing diapers.