For example, a band like Joy Division. Two masterpiece albums in the form of Unknown Pleasures and Closer, and the untimely death of Ian Curtis cut it all short. They were even heading into the direction that New Order eventually went in, and it would have been interesting to see what Ian Curtis would have done if they fully made the leap into electronic music while he was still alive.
Acid Bath. Their bassist died in a car accident after they released two of the best, most unique metal albums of all time imo. I still listen to them regularly. Sammy went on to do Goatwhore. The vocalist had other projects I wasn’t fond of. But still one of the best bands of all time for me.