I pick Buddy Holly. Honorable mentions to Ritchie Valens, Jim Croce, and John Lennon, but I think Holly would develop in a direction I’d appreciate more. Hypothetically, I think Holly would have changed how the British Invasion impacted American music. American music was too stale compared to what the British acts were doing. I think Holly was the exception to that and, had he been alive, American music would’ve been in a different place in the early 1960s. The Beatles were always going to be a major influence on music in America, but it could’ve gone both ways.
Not Tim?
Probably not if thirty years of potential accomplishment is implied…
…pretty much for those reasons. I wish everybody struck down in their prime had lived and had longer careers. But I can’t really imagine thirty years of sustained accomplishment for most. Like almost all. I don’t think anyone I’ve seen anyone cited yet who I think had thirty years of quality work left in the tank.
But if I could, I’d vote for those who went youngest. I mean if he got his head straight, someone like Nick Drake who died in his mid-20’s (and was gone mentally and had stopped working well before then) maybe could have pumped out another ten or twenty albums of introspective folk music. Some of it good, some of it I’m sure kinda mediocre. But it would have be nice to give him the chance to try.
“Invented” is a tall order.
From the Influence section of the Wikipedia article on Holly:
John Lennon and Paul McCartney saw Holly for the first time when he appeared on Sunday Night at the London Palladium .[86] The two had recently met and begun their musical association. They studied Holly’s records, learned his performance style and lyricism, and based their act around his persona.
Dylan referred to this in his acceptance speech when he received the Grammy Award for Album of the Year for Time Out of Mind in 1998: "… when I was sixteen or seventeen years old, I went to see Buddy Holly play at Duluth National Guard Armory and I was three feet away from him … and he looked at me. And I just have some sort of feeling that he was … with us all the time we were making this record in some kind of way.
Mick Jagger saw Holly performing live in Woolwich, London, during a tour of the UK; Jagger particularly remembered Holly’s performance of “Not Fade Away” – a song that also inspired Keith Richards, who modeled his early guitar playing on the track. The Rolling Stones had a hit version of the song in 1964.[94] Richards later said, “[Holly] passed it on via the Beatles and via [the Rolling Stones] … He’s in everybody.”
Clapton recounted the first time he saw Holly and his Fender, saying, “I thought I’d died and gone to heaven … it was like seeing an instrument from outer space and I said to myself: ‘That’s the future – that’s what I want’”
By the same token, I’d add Beethoven.
Instead of dying at 56 (at the peak of his creativity), imagine if Beethoven had lived 30 more years—healthy, feisty, and still shaking his fist at the world. Had he lived into his 80s (til 1857), he would’ve overlapped with Liszt, Berlioz, and early Wagner. It’s exciting to think he may have composed a dozen or more symphonies, dived deeper into wild string quartets, and pushed classical music into uncharted territory—possibly inventing modernism (before modernism had a name). Who knows—he may have even topped his seemingly untoppable Symphony No. 9, arguably the greatest piece of music ever written (it’s certainly my favorite). He was already sketching a 10th Symphony, and his late works showed a mind exploding with ideas. We might have had a century shaped almost entirely by him.
Something that often gets brought up when discussing new hires: “Sure, they’ve got twenty years of experience, but is that twenty years of pushing themselves and learning new things? Or did they just have one year of experience twenty times?”
I could get snarky and bring up performers that, had they lived on, would’ve just been doing the same schtick for three more decades. But you know who they are…
Mine too, he was just hitting his stride and would have had a long career ahead of him. He might not have been huge, but I can see him as someone like John Denver with his story telling and such.
Phil Ochs. He would have hated it, but it would have been great to have his voice around through the Reagan years and the post-9/11 madness. (And today, but you did specify a thirty-year time frame.)
That’s why I thought of Gershwin. Later in life he was starting to move beyond popular song form. And had the classical training to orchestrate larger scale works.
He might have been on a trajectory to rival the great classical composers, if he had lived…?
Then I’ll vote for Karen Carpenter. At the end of her life, her manager/brother was sidelined for treatment of his addiction problem, and she turned to Phil Ramone (of all people) to produce what turned out to be her one and only album without Richard Carpenter. The record label sat on it, despite no less than Quincy Jones expressing interest in remixing it. If she could have gotten away from the easy-listening crap she had been forced to record, she could have had a long career as a song stylist. Maybe not a superstar, but a super -successful cabaret act with a loyal following.
Weird Al
And this is why I asked whether the OP really meant just “thirty more years of performing.” Why waste my choice on someone like Mozart or Beethoven or Gershwin (or Schubert or Mendelssohn or any of the other composers who died too young) if it just meant they kept performing longer but didn’t write anything more?
Just performing. If they write their own stuff on the side isn’t covered by this hypothetical.
Certainly also deserving, but overall Jeff has had the greater emotional impact on my life (as much through accident of timing as through his music, I guess), so that’s who I went with 
Maybe we should have a parallel thread about composers, then?
Perhaps it’s because I’m a musician and songwriter myself, but I find I’m more interested in artists who write their own material versus those who are essentially just performers…
Of course Gershwin famously played at society parties all the time in NY, apparently.
Tom Lehrer.
And he died just hours after I posted this. 
That faint sound you hear in the background is probably me repeating “Damn it, damn it, damn it” over and over again.
Please don’t post anymore in this thread. 
INXS’ Michael Hutchinson (sp?)