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.
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.
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.
The 'Harry Potter' books are set in a magical world full of its own stories, prophecies, and secrets. Sirius is a complex character with a lot on his plate. He has to deal with his family history, his time in Azkaban, and his loyalty to his friends. Fanfiction is a form of creative writing by fans outside the original story. Sirius doesn't have the time or the inclination within the context of the story to be reading fanfiction. His life is too filled with real magical drama and danger.
One great fanfiction about Sirius' childhood could be one where it delves into his relationship with his family. Maybe it shows how he started to rebel against the pure - blood mania in his family at a young age. For example, he might have befriended a Muggle - born wizard secretly, which his family would have abhorred.
Well, if Sirius read fanfiction, it could open up a whole new side to his character. Maybe he'd start to question some of the things he thought were absolute in the wizarding world based on the creative ideas in fanfiction. But realistically, he has so much going on in his life - from trying to clear his name to protecting Harry - that it's hard to imagine him actually taking the time to read fanfiction. He's more of a doer in the real magical world than a consumer of fan - created fictional works.
No. Liches are often associated with dark and unethical magic. Harry has always been on the side of good and learning from a lich could corrupt his values.
Zelretch is likely a very powerful and perhaps mysterious character in this fanfiction. He could be an ancient wizard or a being from a different magical dimension with knowledge far beyond what is known in the normal Harry Potter universe.