Well, in some fanfictions, Morgan might hate Peter because she could be jealous of the attention he gets from the other Avengers or people around them. Maybe she feels that Peter is taking away the spotlight from her or her family in some way.
John Marston is a complex character. He was once an outlaw but then got caught up in trying to redeem himself. He had a family, and his relationship with his wife and son was a big part of his story. He was forced by the government to hunt down his former gang members. In the end, he sacrificed himself to save his family.
One idea could be that Arthur Morgan somehow gets transported to the RWBY universe. He could use his skills as a gunslinger and survivalist to help the RWBY characters in their battles against Grimm. Maybe he teams up with Ruby and teaches her some of his unique combat techniques.
In most cases, playing as undead John Marston isn't a feature in the story. The game developers likely had other creative directions for the gameplay and narrative.
The duration of John Marston's story in RDR2 depends on how you play. On average, it can take several dozen hours to complete his main storyline and related side quests.
Maybe Tails hates fanfiction because it often goes against the original vision of the character. Fanfictions might portray him in ways that he doesn't like or in situations that are inconsistent with the official story.
Fanfics about Arthur Morgan and Mary Linton can be really interesting. They often explore the complex relationship between the two characters. Some fanfics might focus on their past love and how it could have been rekindled, while others might take a more tragic turn, exploring why they couldn't be together in the end.