John Wick is indeed fictional. It's a part of the imaginative world of movies, filled with exciting and often over-the-top action sequences that are designed to entertain rather than represent real events or people.
For John Wick fan fiction, you could write about a scenario where he loses his memory. This would be really interesting as he'd have to rediscover his skills while also being hunted by his former enemies. Or perhaps a story where he has a protege, a young person he takes under his wing and teaches the ways of the assassin. Also, a fanfic could focus on the backstory of some of the unique weapons he uses, and how he came to acquire them.
John Wick isn't regarded as science fiction. The movie series is known for its realistic, contemporary world and raw action sequences rather than anything related to science fiction themes.
First, re - watch the movies to get a good feel for the character of John Wick. Notice his mannerisms, his fighting style, and his code of honor. Then, come up with an interesting concept. It could be something as simple as John Wick having to face a new type of enemy. Next, create a basic plot outline with a beginning, middle, and end. Don't worry too much about grammar and style at first, just get your ideas down on paper.
Sure. There are John Wick comics available. These comics might delve into untold stories or provide alternate takes on events from the films, giving fans even more to enjoy in the John Wick universe.
Definitely not. John Wick is purely fictional. The movies and the character are creations of the entertainment industry, not based on real events or people.