The Scouring of the Shire in The Lord of the Rings is that part of the story that really drives home the way the Hobbit characters have changed over the course of their journey. At the time people wondered whether it was meant to be a comment on post-war Britain, with its rationing and all. But Tolkien refuted this in the second edition's Foreword:
It [the Scouring] is an essential part of the plot, foreseen from the outset, though in the event modified by the character of Saruman as developed in the story...
I know there's some detail about early versions of the chapter where Frodo was meant to fight Sharkey (possibly Lotho, at the time) in Sauron Defeated (though I've not read this myself), but how "foreseen from the outset" is the Scouring really? Do we know how old the idea is? Saruman's part is not original, as stated, particularly as he only arrived in the writing of the story after several waves, it seems in 1940. But I have a hard time thinking that the plot was worked out all the way up to the Scouring before Saruman was introduced, since Fellowship was being written and rewritten multiple times, and many of other main characters hadn't apparently turned up yet (famously, for example, Faramir). Even the Ring itself as the crux of the story wasn't there at the outset.