Alain Lemieux actually put up 34 points in 42 games with a pretty bad Blues club in 1982-83. Then when he went to Quebec in 1985, he had 22 points in 30 games (he was getting power-play time). He was a slightly stockier 'Gretzky-size', though, not a 'Mario-size', and maybe he didn't like the NHL grind or physicality or something. But the stats (and bloodline) do suggest he had the talent to play in the NHL if he'd been totally committed, etc.
Brent Gretzky did play in the NHL (incl. one game vs. Wayne), but briefly. He and Keith were small.