Douglas Adams… because I think his voice and perspective was unique, insightful, and important, the first philosopher of the internet-age.
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Can you imagine how much he would have loved all of this cloud internet/Twitter/iPad/remote charging/cyborg cockroaches shit? 
Jack Vance, who is now blind and in his nineties and probably has many outstanding fantasy and science fiction stories still inside him.
Second choice: George Macdonald Fraser who hinted at numerous Flashman stories yet to be told.
If by “Bach” you mean Carl Philip Emmanuel, then maybe. At the time, CPE was way more popular than stogy JS.
I’d agree, assuming that he used the 20 years to return to actual writing and creating and not sort of existing as a pop philosopher and self-important being. Success seemed to… derail him.
Christopher Lee. All the great work that he’s done and he just now seems to be hitting his stride.
If I could go back in time, then the obvious answer would have been Jimi Hendrix.
Jim Henson. A billion kids around the world owe their upbringing to Jim Henson’s creations. He should have had two more decades to create wonderful things.
Leonard Cohen (but only if he gets to keep his ‘Old Man Cohen’-voice).
Jimi Hendrix. He’s the greatest musicain of the 20th Century, IMO.
Huh, really? I was always under the impression that JS was the big honcho and that his kids sort of rode his fame, the way celebrity kids do their parents’ these days.
Half-ass cite: BBC - Radio 3 - Feature
My parents, especially my mother, are big fans of Rumpole of the Bailey, both the books and TV series. So probably Leo McKern, because he passed away before he could film all of the Rumpole stories/books. Otherwise, John Mortimer, so he could write more of the books/short stories featuring Rumpole.
Dame Judi Dench. She’s 80 now and while still going strong, who’s going to replace her when she retires.
If we get to pick a dead person, I’ll suggest Bob Fosse.
Todd Rundgren, if only because he’s nearly 65 and is not content to rest on his laurels, but endeavors to make each new album and tour unique, constantly exploring new genres. Maybe, in enough time, the damn radio programmers will take a listen and figure out that he’s a lot more than “the guy who did Hello It’s Me and Bang The Drum All Day” and play some stuff off his newer albums.
As his mom is still around, it looks like he has the genes to have another twenty productive years on his own.
I second this. (Though technically he never abandoned his career – he’s had a recurring role on The Good Wife this year – he just scaled it back.)
If reviving the dead were allowed I’d pick Isaac Asimov, but since it isn’t, I’ll go with Harlan Ellison.
Half-ass, indeed! 
Most of those quotations are by composers and others in the later 19th century. JSB in WAM and LvB’s day was certainly well respected, but they probably only knew a small amount of his keyboard music. It wasn’t until the late 1820s that JSB was truly venerated. JSB was the third choice for the job he held in Leipzig for the last few decades of his life. One of the people on the “search committee” said something like “since the best men aren’t available, Bach will have to do.”
You might find this of interest.
John Lennon.
I think he had a lot of influence and could have made the world a better place.
He still won’t deliver The Last Dangerous Visions.
Hmm…well, my first choice would have been Steve Jobs (I could argue he was more than a bit of an artist, but I guess he falls more under “scientist” making him ineligible).
My other choices:
Stephen King: He went through a period where he was writing the purest crap there for a while, but he’s hitting his stride and starting to produce some quality stuff again. So I’d like to see what he comes up with given another 20.
Alice Cooper: The guy’s 64 and he runs around like a guy half his age, touring and loving his fans as much as they love him. I want him around as long as possible.
Eric Woolfson: The other half of the Alan Parsons Project, who left too early at 60. Bring him back and let him keep coming up with more stage musicals based on old APP albums.
I would have said Neil Peart from Rush, but unless I can zap Alex and Geddy too that hardly seems right.
I’d have to go with Stevie Ray Vaughan, who had finally gotten clean and was making the best music of his life when he died.