It varies. Some spinoffs might build on the main canon or offer alternate perspectives, but not all are strictly considered part of the core JoJo universe. It often comes down to how they tie into the main plot and characters.
Yes, Riverdale is indeed a spinoff from the Archie Comics. It takes inspiration from the characters and settings of the comics and puts a modern and darker twist on the story.
For a spinoff to be good, it should have a unique plot that ties in with the original but also offers something new. Also, maintaining the tone and style of the original is important.
First, you need to have a deep understanding of the original story and its characters. Then, come up with a fresh and compelling idea that expands the universe without overshadowing the main plot.
There could be various fanfictions. Maybe some that focus on alternative endings, like ones where certain characters survive who didn't in the original. For example, a fanfiction where Erwin survives and leads the Scouts in a different way.
One way is to check on popular fanfiction websites. Look for ones with high ratings and a lot of reviews. For example, Archive of Our Own (AO3) has a large collection of AOT fanfictions. You can search for the most popular ones there.
Definitely not. AOT is a work of fiction that combines elements of fantasy, action, and drama. It's set in a made-up world with its own unique plot and characters.
The AOT manga has finished. It wrapped up all the major plotlines and provided a conclusion to the story that fans have been following for a long time.