If we are talking Literary works,
The Silmarillion was originally not intended to have much to do with
The Hobbit and the references to
Gondolin were originally a sort of nod only to
Tolkien's greater
Cosmology.
But when he was explicitly asked for a sequel to
The Hobbit and his earlier works were rejected as being too cerebral and abstract (which I always find puzzling, as I am as captivated by
The Silmarillion as I am by any other of Tolkien's works), then as he began
The Lord of the Rings he discovered the connection to the pre-History of the
Elves through its writing and creation; a serendipitous event that has contributed to, and enriched, the lives of many from that time forth.
Tom Shippey's claim, in his Tolkien Biography title:
The Author of the Century was no boast. Tolkien, along with other greats from the early Days of SciFi and Fantasy, stood out from the crowd as having created something so original that it would create its own genre, of which it remains the sole work of note.
What is sad is that in the days of his leisure, after having become very comfortable from sales of the book, he became fearful of adding anything of real substance to the mythology, cosmology, and history of
Middle-earth.
Much of the minutia he produced during this time is itself remarkably interesting (genealogies, heraldry, paintings, . . .), but he balked at the hard work of unifying and correcting aspects of the early history (such as expanding the
First Age several thousand years to account for the tale of the
Fall of Men in the
Athrabeth Finrod a Andreth.
When I was at the Bodleian Library in the 1980's, I did see one product of this period that he kept coming back to (and which I notice is conspicuously absent from
The History of Middle-earth regarding the problem he had regarding the cosmological, theological, and philosophical problem of the
Orcs. That he
HAD considered having them bred in the ground like worms, but rejected the idea as "to hideous to contemplate" in a fashion similar to his rejection of the beer stein he was given with the inscription from
The Ruling Ring inscribed around it.
Tolkien so believed in the reality of
evil, and that even in fiction the Dark Lords were a real manifestation of Luciferian forces that he never used the mug for anything other than an Ash-Tray.
Tom Shippey points this same problem out, in that Tolkien was hampered by his own beliefs in the manifestation of Good and Evil in his own works to be able to progress on them as another author might.
MB
I obviously take Middle-earth too seriously!