He was also starting to produce other artists’ works, wasn’t he? He might have gone on to not only be a star in the performance sense of the word but also in the creative/technical sphere.
Well from a sales perspective, in the US he seemed to be past his peek. He had a huge hit with That’ll Be The Day in 1957. Heartbeat, his final single of 1958 got to #88 on the Billboard Hot 100. You can argue that he might have been in the middle of a career slump and would have recovered to greater things in '59 or later had he survived but the trend was downward before he died.
The original Sonny Boy Williamson was killed by a mugger at the age of 34. Another guy named Aleck “Rice” Miller appropriated the name to become the second Sonny Boy Williamson.
Hitler peaked in 1942.
A lot of these people peaked after they died, so I guess you could make an argument that they didn’t die at the peak. However you’re not really wrong either, 'cause you could make an argument that had they not died, they wouldn’t have been so successful.
Then there’s borderline cases like Freddy Prinz who’s career was actually going down at the time of his death. His show was only a big hit the first year, and declined each year substantially each year after. He was still very famous, but not at his height. But I’d give you that one.
Interestingly enough, to hijack this a bit, one could make a case that death brought Janis Joplin and Jimmi Hendrix status they wouldn’t have had. Look at their contemporary and more popular Sly Stone (Of Sly and the Family Stone). His success at the time was equal to those two but because he stuck around he declined into nothing. Janis Joplin could’ve easy fell into a “one-hit wonder,” status as could’ve Hendrix, had they lived or evolved into a “living joke” as Keith Richards has.
The previously mentioned Ian Curtis. Joy Division were about to embark on their first US tour, they’d garnered critical acclaim and attention, and he hangs himself the day before they’re set to leave. 
Unless this was a double whoosh… He was a “tightrope” performer.
Henry Kuttner was just starting to get notice as a Sci-Fi author when he died suddenly of a heart attack in 1958.
I suppose you could add Payne Stewart to this list if we are including athletes. Killed in a freak plane crash four months after winning the US Open.
Has Jim Henson been mentioned?
Ayrton Senna
I’m one of his biggest fans, and I cried like a baby when I heard he’d been killed, but I think it’s fair to say he had peaked by the time of his death. I don’t think he was going to win another three F1 world championships.
I would think his peak success would’ve been the (first two) Superman films, which were, what, over a decade before even his accident.
If the following was already mentioned, I missed it:
John Candy
John Ritter, possibly.
Maybe Three’s Company was the height of his career, but it seems 8 Simple Rules was doing well in the ratings. I have no idea if it equaled Three’s Company or not.
Sylvia Plath.
Diane Arbus.
Bluesman Robert Johnson. Dead at 27. Clapton popularized Robert Johnson’s original "Crossroads."Wikipedia entry here: Robert Johnson - Wikipedia
River Phoenix died right around the time of his role in the third Indy movie, and had he not passed, we might have been able to avoid the antics of his hack brother, Joaquin.
Seconded. The first person that came to my mind. After a successful run on SNL, he had a popular role on NewsRadio, a recurring guest role on Third Rock from the Sun, and did some Simpsons voices.
If you’re talking about artistic achievement, yes. If you’re talking about commercial success, no. It wasn’t until many years after his death that Robert Johnson became famous and his recordings sold in enough quantities to make real money. I suppose he’s like Van Gogh in that regard.
Gilles Villeneuve
John Belushi
Bill Hicks
Warren Zevon. Although I think Zevon’s best years artistically were in the 1970’s and 1980’s, his last album (“The Wind”) was recorded while he was dying of cancer, and it went on to win two Grammys and outsell any of his previous albums. Also, I think at the time he died he was at the absolute peak of personal popularity in the sense of people knowing who he was and respecting his music.
He might have become well-known if he’d lived just a bit longer; he was at the height of his powers.
John Hammond wanted Robert Johnson for his “From Spirituals to Swing” show at Carnegie Hall in 1938. But he’d just died, so Big Bill Broonzy got the job.