Alongside Cevert and Pryce, I’d add Bellof and Rindt to the list of drivers who could have achieved more had they not lost their lives to soon.
As for poor career choices, Emmo could have won more titles had he stuck with McLaren as opposed to Copersucar. His IndyCar successes proves that he could have become a triple or quadruple champion with McLaren. Timo Glock made a similar poor decision, turning down a drive with Renault in favor of Virgin/Marussia.
Karl Wendlinger was once regarded as a bright prospect from his Mercedes days with Michael Schumacher. His crash in Monaco virtually ended his Formula 1 career and shook his confidence. The Brazilian duo of Ricardo Zonta and Antonio Pizzonia were also highly-rated in the junior formulae but never really succeeded in F1. Likewise, the infamous duo of Vincenzo Sospiri and Ricardo Rosset were successful in F3000.
Ukyo Katayama was very quick in 1994 but was let down by reliability. It was rumored he was offered a drive at Benetton but turned it down due to his cancer, marking another wasted talent. In a similar and more recent scenario, Dean Stoneman looked set for a bright future in open-wheel racing until his diagnosis of testicular cancer.
There are plenty of talented drivers in the junior categories who couldn’t or can’t graduate to F1 or IndyCar due to budget constraints. The most obvious examples in my mind right now are Albert Costa and Luca Filippi.
But I think a really obvious example of a wasted talent is Derek Warwick, who was snubbed from Lotus by Ayrton Senna. After impressing at Renault, Warwick could have won races in the Lotus 98t but is now regarded as a what-if.