The entire point of fiction is to create a story. Nonetheless, it needs to be convincing. It may have never happened, but the reader needs to believe it did or at least could. When a story isn’t credible, it becomes one of those items mentioned yesterday that throw you out of the story, a huge faux pas.
Create any possibility you want, just make sure the reader will be convinced. Characters need to be believable, situations credible (no matter how extreme), and plots convincing. This is the fun of creating a world, making it real. If it’s real inside your head, that’s only part of the battle. You need to convey enough detail to your reader for them to believe it and envision it, too.