Can you clarify whether, in your hypothetical, they would just have thirty more years as a performer, or whether they would also have more time as a songwriter/composer/recording artist? That is, would we get new music from them?
(As I look over the responses so far, some people seem to be interpreting it one way, some the other.)
Definitely the former, perhaps the latter.
I was debating between Karen Carpenter and Badfinger…but I agree with you.
Johnny Cash, but it’s not just about the music (if he were still alive, he’d be ancient). Rather, I think that his voice could have had a positive effect on the way our country has developed, as a champion of the common man that the common man actually listens to and respects.
I really don’t think his career was cut before his prime.
Your OP doesn’t specify that they had to be cut down before their prime, just before their time. Though even so, Johnny Cash is debatable.
I’m leaning toward John Lennon, who was probably past his prime but whom we nevertheless lost way too young. Especially since another 30 years of Lennon would open the possibility of him performing with McCartney again.
Then let me clarify, please.
I am looking for people or groups that were stopped before they hit their true greatness, not people that had long careers but might have been able to squeeze out yet another spurt of greatness in their old age. Lennon, Cash, Garcia-not really. Cyrus and Swift-still going on strong, so they don’t count either.
That makes it a lot harder to judge: how are we to predict possible greatness from a limited early catalog? For example, Buddy Holly had some good songs, but would he have evolved into a really great world-class songwriter over the next few years?
I can think of one example that is not in music: Evariste Galois. Who more or less single-handedly created mathematical Group theory in his late teens, then died in an apparently pointless dual at the age of 20.
Just asking for opinions.
Of course, no worries: we are just shooting the breeze here… 
For example, Hendrix did things on guitar that nobody had done before (and arguably nobody has quite done since)? And his songwriting was very focused on being a vehicle for the guitar work, I’d say.
But where might he have gone from there? We will never know.
Robert Johnson
Of course we will never know. That should have been implied by the word “Hypothetical” in the title…and the fact that we cannot go back in time to prevent death and rewrite history.
Oh, just wait till I get the convolution inductor back into working order on the ‘Alternate-Universe-O-Tron’ in the basement.
Unfortunately the nearest supplier is in Alpha Centauri, and apparently they are back ordered from the main factory on Trantor…
Care to pick one?
I can’t decide between Nick Drake and Jeff Buckley. Despite dying way before their time (at 26 and 30, I think) their music to this day has had a lasting impact and influence on artists past and present. Would’ve been amazing to see what they could’ve done with more time.
Kurt Cobain.
Ian Curtis was the other possibility, but in the end Kurt was an easy first choice.
Fair; let me expand on my opinion on Freddie Mercury a bit, then.
When he died, in 1991 at age 45, Queen had been going for 20 years, and, arguably, they had probably peaked in the mid-late 1970s. But, Live Aid in 1985, when he and his bandmates were all in their mid-to-late 30s, showed that they were still on the top of their game, from a performance standpoint.
They were never a formulaic band, and their music ranged across a lot of different styles and influences throughout their career, in no small part due to Mercury’s interests and writing. They also kept creating new music; in fact, their output rose in the late '80s, when Freddie realized that he didn’t have much time left, and he and the rest of the band strove to create as much music as they could in a limited time.
Had Freddie not gotten ill from AIDS, that burst of creation likely doesn’t happen, but I do think that they would have continued to record regularly, and tour regularly, well into the '90s, if not beyond, and that Freddie would have continued to write new, interesting music.
Harry Chapin. Died at 38, so maybe another 30 years of writing and performing. Also probably a better human being than many listed above so there’s that too.
My first thoughts were Chris Cornell or Layne Staley, but they arguably were very successful before their deaths. I’d still love 30 more years of their work, though!
Maybe Jim Cherry? He was an original member of Strung Out and Pulley (both bands are still active) and may have done some interesting things, but he died suddenly of a congenital heart disease at the age of 30.