This is an easy one for me: Jim Croce
Jane Austen who was 42 when she died.
I’d have like to see Humphrey Bogart live to see his kids grow up.
Dale Earnhardt
After his death, major changes were made as far as safety, such as soft walls and safer cars, but he was one of the few whose voice was paid attention to by the France family. I wasn’t a fan, but he could possibly have prevented The Chase, the Lucky Dog rule, the no racing to the finish line rule, and several others that have ruined a lot of the races.
If we’re doing politicians/public figures, then I would add:
Paul Wellstone (one of the loudest anti-war voices, silenced far too soon)
Martin Luther King, Jr. (obviously)
Franklin Delano Roosevelt (would he have dropped two bombs?)
Abraham Lincoln (reconstruction would’ve been very different)
Gandhi, Itzhak Rabin, Anwar Sadat…
It’s really hard to pick just one. I think I’d actually go with Rabin or Wellstone.
Since SRV has already been posted, I’ll say Jeff Buckley.
I’d be happy to cause Nancy Grace’s, however! (but only by accident 
)
Another vote for Douglas Adams. I would have loved a 15 book series of Dirk Gently!
The Two Ronnies. The world needs more laughs from them.
Douglas Adams is the best answer of those presented.
If I think of a better one, I’ll post again.All of you mentioning Tolkien may want to read this post. It seems, if he’d stayed alive, a lot of what you know about LOTR and Middle Earth would have been totally changed.
Keith Richards.
Man, I’m good. 
John Paul I.
This is who I was going to pick as well. That or Phil Lynott.
Edward the Head beat me to Phil Lynott, so I’ll say Davey Allison instead. He was my last great childhood hero.
And he probably doesn’t really count as a true celebrity, but I wish there had been a way to prevent Dave Prichard from Armored Saint from getting leukemia. Loads of talent and reportedly a nice guy to boot.
Probably Bill Hicks. I still remember opening the pages of that week’s New Musical Express and being so shocked to read this short paragraph buried down in the week’s news column that he’d died. I was a huge fan of his work and had no idea at the time that he was even ill. 
Saw it already - hey, if that’s what the Professor really wanted to do, who am I to stop him? But I’d really have preferred that he write new stuff rather than endlessly go back, mull over and rewrite his old stuff.
Jane Austen is a good answer, Dangerosa. Even given lifespans in her era, she died pretty young, and could easily have had some more great books in her.
Mitch Hedberg. I always wanted to see him live, but he died before I got a chance to. And he was way too young.
Also he looked a hell of a lot like John Denver (to me anyway)
And Face? Thanks for the John Denver/Cass Elliott video! It and your nick got put on my blog!
Quasi
You big meanies stole my original pics, John Denver, Jim Henson and Jeff Buckley.
So I’ll pick a girl. Karen Carpenter. She was young and far more talented than she had shown so far. From what I understand, she was a very good drummer.
Another vote for Bruce Lee. He was an amazing cross-genre talent (acting, martial-arts, philosophy) who was just hitting his prime and whose best work was probably still ahead of him.
Natasha Richardson just because she was a mom. I liked all her movies and lusted after her husband, (along with every other female on the planet), but I’d give her son’s their mom back if I could.
Heath Ledger
This list includes spiritual giants & so many taken far too young. But I’d like to add a musician who died almost 10 years ago, at the age of 58.
Doug Sahm–The State Musician of Texas; more information here.
In his intro to the T-Bone Shuffle, he referred to “the Annals of Texas Music.” He held those annals in his heart & soul, throughout his long & amazingly varied career. He might have been a bigger success if he’d just played one style of music. But he kept on playing Tex-Mex Country Cajun R&B Rock & Roll–because he loved the music & wanted to share it with the world.
(OK: An Honorable Mentions for Lowell George. He killed on guitar & wrote insanely great songs. And sang so sweetly.
And for Tim Buckley. He went from starry eyed folky to possessed shaman–with a promised future greatness. Yeah, he was Jeff’s dad.)