No, I got the bit about fiction. I just finished reading recent science fiction that utilizes very similar plot lines in several distinct stories, that's all. Even in the movie "Merlin", Mab's existence was contingent on belief. I think American Gods by Neil Gaiman is pretty much precisely that as well. To quote from its Wikipedia page:
The central premise of the novel is that gods and mythological creatures exist because people believe in them (a form of thoughtform).
So as I said, yes, you are quite right, it is (or can be, a premise for a good science fiction/fantasy novel. And has been, several times. And quite a number of bad ones as well. It's a very interesting philosophical/ontological problem -- does belief precede or follow existence?
Personally, as a physicist, I have to say that correct/best belief (as opposed to fantasy) follows existence, and ultimately is empirically founded on it. Religion is the other kind, the one where there is plenty of belief but no solid evidence for the existence of the objects of the belief.
To paraphrase Austin Power, I may believe in a gold plated potty but that doesn't mean that one exists, not even in an infinite Universe where it is possible that one exists.
Or, to toss in a reference to Russell, it is possible that there is a silver teapot floating around in an orbit around Saturn. I might, if I were un-sane enough, believe in such a teapot, and write out an entire mythology based on the teapot and how it got there and what its existence means for the Universe in general. But even if I convinced every human alive on Earth that I must be right, that as an authority on all matters concerning the Sacred Silver Teapot my word on this cannot be doubted, not even the deep and profound belief of every person in the world has the slightest chance of either creating the teapot from nothing but the belief itself or increasing the probability that the teapot exists from a number so close to zero that their kids go to the same school, noise from parties in zero's yard keep the probability up at night, they argue about who is going to trim the hedge and who is providing the beer...