It's been going on a while: Stoppard's Rosencrantz and Guildenstern is a
spinoff of Hamlet, of course, and Hester, a prequel to the Scarlet Letter,
came
out a few years ago (includes all the good stuff that happened before
Hawthorne's story.
It may be part of the alternate history trend.  Mine has Huck stow away to
Mexico with Scott's army and be informally adopted by RE Lee--then wind up
on
the opposite side in the Civil War. It was lots of fun. I always wondered if
maybe Twain didn't want to go there because it's what lay down the road for
his
characters.  (In mine, Huck has a chance to change history.)
I've wondered what Horatio's story would be like as a sequel to Hamlet...
oh
well, papers to grade.

Dan Walker