Sure. Some female Jonathan Joestar fanfictions might explore how the character would be different if she were female in the context of the original story's events. For example, how would her relationships with Dio or other characters change? Check out fanfiction.net as well, as it has a wide range of such works.
One popular crossover is with the Fate series. Combining the unique abilities in the Fate universe with Jonathan Joestar's Hamon powers can create really interesting storylines.
One popular Jonathan Joestar crossover fanfiction is 'Jonathan Joestar in the World of Harry Potter'. It's really interesting to see how Jonathan's noble and brave character would interact with the magical world of Hogwarts. He might use his Hamon skills in some unique ways within that universe.
Another place to look is FanFiction.net. There are many fans who post their Jonathan Joestar crossover fanfics there. However, the quality may vary, so you might need to read the reviews or summaries first to find the good ones.
Well, one really creative idea is a crossover with 'Alice in Wonderland'. Imagine Jonathan falling down the rabbit hole and dealing with the strange creatures there using his Hamon. It's a really out - there concept that combines two very different worlds.
Guts is purely fictional. It's a concept or character that exists within the realm of imagination and creative storytelling rather than being based on reality.
It depends on the specific publisher and the official stance. Sometimes it might be canon, but other times it could be more of a side story or alternate take.
I'm not sure exactly who Jonathan Wamback is without watching the story. He could be an ordinary person with an extraordinary story, or someone well - known in a specific community or field.