I had no idea that Tolkien ever considered a sequel to The Lord of the Rings, let alone one that had multiple revisions by the time of his death. This video into that potential book, with a draft title of The New Shadow, is a fascinating look into what could have been, but more so, it's a perfect example of why Tolkien's work is so timeless: as an author, he had a particularly impressive grasp of human nature, and a poetic turn of phrase that allowed him to express it in ways few others can manage.
It also hints at an inversion of one of the few criticisms often levied at LotR, which is that it is almost too optimistic about the goodness of men and too simplistic in its portrayal of good versus evil. I'm not sure I've ever fully agreed with that take (not with the books at least; the movies may warrant it), but the very fact that a sequel would have considered how "easily sated" Men are with peace and "good" (to paraphrase his comments), and imagined a world in which young Gondorians would have played at being Orcs or followed a (potentially) Black Numenorean into a new age of darkness, well, that's some pretty bleak and harrowing stuff. And a little too close-to-the-bone in modern times, with the rise of the Alt-Right and resurgence in Nazism and fascism more broadly. Honestly, there's a lot you can point to in Tolkien's work that can be explained by his lived experience with the Great War and WWII, but I think that this plot and the time in which it's set (within living memory, but only just, of the fall of Sauron) is too predictive to be anything other than genuine insight and understanding, which is cool, albeit terrifying.