HoMe XII - The Peoples of Middle-earth contains the beginnings of a story set in the Fourth Age, but it was soon abandoned. Tolkien said he felt the story would be 'nothing more than a thriller'. The ancient nobility and majesty of Middle-earth's peoples passed with the Elves, or perhaps later with the death of Aragorn and Arwen. Middle-earth was simply not 'magical' anymore.
He obviously felt that there was not much more to say after the War of the Ring, and that's kind of how I feel.
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