The Black Panther comic has its own unique fictional universe and narrative that is not related to the Black Panther Group. The comic's creation was based on comic book concepts and storytelling rather than any direct connection to the group.
One great 'Black Panther' fanfiction is 'The New Wakanda' which explores a Wakanda that has to face a new, unseen threat. It has great character development for T'Challa and his sister Shuri.
The Legend of Black Panther is an audio novel that can be listened to for free. However, the search results did not provide clear information on which platform could be listened to for free. Therefore, I don't know the specific platform for free listening to books.
Perhaps the fanfic could be set in a post - Endgame world. Black Widow is somehow brought back and she teams up with Okoye to retrieve a powerful artifact that has fallen into the wrong hands. They travel across different realms, facing various enemies. Their different personalities, Black Widow's more sardonic and independent nature and Okoye's loyalty and discipline, would lead to some great character interactions.
It's very important as it gives the backstory. It shows how the Black Panther character and the Wakandan nation came to be. It sets the foundation for the character's values, powers, and the unique culture of Wakanda.