There have been good prequels. There have been functional retcons. Not everything is Greedo shoots first and pushing death sticks.

The Tarantinoverse, or w'ev, of the bulk of his movies and scripts, is the way it is by the late 20th Century and early 21st, because Hitler died violently trying to watch a movie. That's good stuff. That helps contextualize a whole slew of things you otherwise wave off as author-obsessions or because it was fucken cooooool!

Fire Walk With Me or the Utena movie as (potential) wraparound/sequel/prequel to their TV counterparts illuminate the original work substantially. I'd go for the audio and prose prequels to Twin Peaks, as well, though not so much the pre-show bits of SKU as seen in the comics' first chapters.

The stuff with flailing Jerry at home with his mother and sister, in the later books of Michael Moorcock's Cornelius Quartet really substantiate the earlier trying too hard to be cool version of the character. Funny to see Prince riff on that, intentionally, with Purple Rain and Graffiti Bridge, and come out pretty powerful with that notion of a the slum kid dreaming, but in Prince's movies, we get it right in the same package, and in the Cornelius works, it built up over years of releases and then oh, well that makes sense.

Spider-Man as victim of molestation as a child. Sooo many fans hate that, it was always noncanon (though Kevin Smith tried to bring it in), but for me, it makes sense, it works, and it's got tons of potential power and relevance.

Any others? Let's stay positive.