A unique feature is the new relationships that are formed. Harry's connection to the Black family can lead to different friendships and rivalries. In some fanfics, he might become closer to certain Black family members who were previously on the periphery of the main story, like Sirius's more distant relatives.
You can start by looking on fanfiction.net. It has a large collection of fanfics and you can search specifically for 'lord harry james potter black fanfic'. There are likely to be many stories there.
There are fanfics in this category that focus on Harry's growth as 'lord harry' in a more magical and political way. They might show him dealing with the wizarding world's power structures and how he uses his new status to bring about change. These often involve him making allies and enemies in unexpected places.
Well, 'Harry Potter and the Black Legacy' is a fanfic you might like. It focuses on how Harry's Black heritage influences his journey at Hogwarts. There are some cool new characters introduced that are related to the Black family, and the plot twists are quite engaging.
One great 'James Potter and Sirius Black fanfic' to start with is 'Prongsfoot: The Marauders' Era'. It really captures their friendship well and has a lot of interesting plotlines that are true to the characters' personalities in the original series.
I'm not aware of any such fanfic. Fanfiction often contains a wide variety of strange and inappropriate concepts, but this one seems rather extreme and goes against the nature of the original Harry Potter story in terms of the characters' relationships.
Perhaps James had some hidden magical ability that he was unaware of until that moment. It could be something that was passed down through his family but had never manifested before. This ability could have protected him from Voldemort's killing curse. Or, it's possible that there was a plot by some other characters to save James. There could have been a group of wizards or witches who had been secretly working to keep James alive because they knew his importance in the fight against Voldemort. They might have used some sort of time - turning magic or a protective charm that Voldemort couldn't break through.
One common theme is family. Since they are father - son, many fanfics explore their relationship and how they bond over their shared family heritage. Another theme is legacy. They both carry the Potter name and the stories often deal with how they uphold or change the Potter legacy. Magic is also a big theme as they both are powerful wizards and the fanfics show their magical abilities in different ways.
One great fanfic could be 'A Father's Legacy'. It delves into James Potter's relationship with Harry through a series of magical adventures. Another is 'James and Harry: Unseen Bonds' which focuses on how James's spirit influences Harry's journey at Hogwarts. There's also 'The Potter Connection' that shows their relationship in an alternate universe where they can interact more directly.
There is a fanfic where James Sirius Potter discovers a hidden mystery in Hogwarts. In this story, he teams up with some new friends. The author does a wonderful job of creating a believable world and the relationships between the characters are really well - written. It's a great read if you want to explore more about this character.