This all may have started a few years back, when the composer Colin Matthews wrote a new ending to Holst's suite "The Planets," called "Pluto, the Renewer."
(Holst wasn't able to write a Pluto piece himself, Pluto having not yet been discovered in the 1910s.)
Richard Rodney Bennett has written more than one piece inspired by Debussy's piece for solo flute called Syrinx. He calls them "After Syrinx"--and that's how they're sometimes performed: first Debussy, and then, after "Syrinx" . . . "After Syrinx."
If you were listening to Michael Barone's New Releases earlier this year, you heard him talk about "Tchaikovsky 6.1": a presentation of Tchaikovsky's Sixth Symphony, with an added movement at the end, by contemporary composer Peter Boyer, in which Boyer reworks some of Tchaikovsky's themes, for a new finale in his own style.
And the conductor of the Tchaikovsky/Boyer is apparently looking to create other such pieces. It's a clever idea. It could give a certain novel twist to concert programs of traditional fare, and provide an obvious opening for contemporary composers to get their work in front of the public.
So there seems to be a bit of a trend here.
Do you have any nominees? Symphonies that call out for sequels? Endings that leave you dangling? Pieces you just don't want to end?
I think this sucks. If I were a dead composer and someone came along and added something to my work, I would haunt them to their grave! Seriously, I do not think this is a good idea. I have heard the Colin Matthews "Pluto" and his style is so different from that of Holst that it really ruins the effect that Holst wrote in "Neptune." If someone wants to compose "inspired by" music and not force it to be a part of the deceased composer's work, that's fine by me.