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.
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.
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.
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.
The relationship could become strained. Sirius might start to view Harry differently, with a bit of wariness. He might not trust Harry as much as before in terms of making good decisions. But it could also lead to a more open relationship where they talk about things they previously avoided, like relationships and feelings.
One of the most creative could be a fictional wandmaker named Elara. In some fanfictions, she uses unconventional core materials like the feathers of a phoenix - like bird that has the power to heal. Her wands are known for their ability to adapt to the user's needs over time.
It's also likely that they go to Romania. Given Sirius' knowledge of the wizarding world, he might know of a safe haven there. In Romania, they could be around dragons and learn about dragon - related magic. Harry would surely be fascinated by the dragons and it could be a place where he can grow stronger in a different kind of magical environment.
Hermione would definitely be a great help. She's smart and resourceful, and could do a lot of research to find evidence. Also, Ron would be there, providing support and loyalty as always.
Neville could be part of it too. Maybe he sees Harry's training and gets inspired to improve his own magical abilities. He could learn from Harry's experiences under Sirius's training. And the Weasley twins might show up to add some mischief and perhaps even create some magical gadgets to aid in Harry's training.
Sirius would probably be shocked at first. Then, he'd likely feel a deep sense of sadness and guilt. He might think that he should have been there more to protect Harry from getting those scars.
Perhaps they would head to America. In the wizarding world of America, there are different magical institutions and a large expanse of land where they could start anew. They might be drawn to the Ilvermorny School of Witchcraft and Wizardry area, to interact with new wizards and witches and learn from a different magical education system. Also, the American magical community might not be as involved in the affairs of the English wizarding world, providing them with some anonymity.