(some spoilers near the end of the post, though I try to be vague)
Not long after completing The Lord of the Rings, J.R.R. Tolkien briefly began work on a sequel called The New Shadow, set 100 to 150 years later during the reign of Aragorn’s son Eldarion. (The main link between the two stories is the minor character Beregond, the noble but disgraced soldier of Gondor whose son, Borlas, would have been a major character in The New Shadow.) The New Shadow reveals that the eucatastrophic fairy-tale ending of The Return of the King was extremely short-lived; with the Elves and the Wizards gone from Middle-earth, the Dwarves moving underground, and the Hobbits now isolated in what amounts to an enclave in the Shire, Men are quickly falling back into their old bad habits. In fact the Men of Gondor already seem to have forgotten much of the details of the War of the Ring, even though it remains in living memory: they seem not to remember, or take seriously, the fact that they once strode with gods and angels in a war against pure evil, and were victorious. Instead, children play at being Orcs for fun; the death of Elessar has been an occasion for political striving and reactionary plots; and even something like a secret death cult of devil-worshipping rebels seems to be spreading through the elites of Gondor.
Tolkien wrote 13 pages of it.
He later wrote:
I did begin a story placed about 100 years after the Downfall, but it proved both sinister and depressing. Since we are dealing with Men it is inevitable that we should be concerned with the most regrettable feature of their nature: their quick satiety with good. So that the people of Gondor in times of peace, justice and prosperity, would become discontented and restless — while the dynasts descended from Aragorn would become just kings and governors — like Denethor or worse. I found that even so early there was an outcrop of revolutionary plots, about a centre of secret Satanistic religion; while Gondorian boys were playing at being Orcs and going around doing damage. I could have written a ‘thriller’ about the plot and its discovery and overthrow — but it would have been just that. Not worth doing.