Writing is an exercise of the mind, and ideas flow best when unrestricted. However, these pleasures are for but a season. Fantastic notions, while exhilarating for a time, soon reveal their lack of substance and thus subside.
Conversely, a work that is entirely realistic will fail to inspire, merely reminding one of their dreary existence. It is once more in the balance between the arcane and the mundane that true literary bliss can be found. That which is fanciful is too light, and that which is practicable too heavy. The latter will crush your enthusiasm, while the former will carry you away.
Certain elements of an unreal story can impart a sense of reality to the work. If the characters are real people with genuine feelings, then the reader will see them as natural even in an exotic world. But this suspension of disbelief is such a fragile thing, and can easily be dispelled by the slightest transgression in the portrayal.
A tale is most compelling when people forget they are reading and immerse themselves in the world you have so lovingly fashioned. To achieve such a state requires believable characters, conceivable actions, measurable results and pleasurable outcomes. There is a balance to all things natural. Your audience should never descry a zipper on the monster’s back.