My view is that the number one mistake that new writers make is trying to write something out of thin air. I totally agree with Sylvia Plath who, per @AdviceToWriters, opined:
Everything in life is writable about if you have the outgoing guts to do it, and the imagination to improvise.
Every novel is some sort of roman à clef or is based on a previous work. For example, Lolita and Humbert appears to have been based on Sally Horner and Frank La Salle. And per Wikipedia, at a minimum, The Lord of the Rings is based on:
[...] philology, religion (particularly Catholicism), fairy tales, Norse and general Germanic mythology, and also Celtic, Slavic, Persian, Greek, and Finnish mythology. Tolkien acknowledged, and external critics have verified, the influences of George MacDonald and William Morris and the Anglo-Saxon poem Beowulf.
Only God can create out of thin air. Consequently, humans must improvise creatively.