To me, Bo Jackson will always be the pinnacle of the dual-sport professional athlete. In the NFL he was an elite running back, and in the Major Leagues, an All-Pro.
Michael Jordan attempted it, but anything short of a multi-year All-Star, would be a failure when compared to his Hall of Fame NBA career. Jim Brown, Walter Payton, and Barry Sanders would have to work really hard to come close to what they were in the NFL. I have a few guys in mind who might be exciting in another league.
Steve Smith: Is at the top of my list. He had the speed, athleticism, power, body control, and toughness to be a dominant player–provided it all translated to another sport. In the NBA, he could have been explosive. I think somewhere between Mugsy Bogues and John Stockton. Maybe even Derrick Rose in his prime. In the MLB, Smith would have been a fascinating player. With his speed, he could have been an All-Star outfielder, a rangy middle infielder, heck, I could see him playing anywhere but first base (it would be a waste of his speed/range). A good comp would be Tim Raines or Ricky Henderson.
Rob Gronkowski: At 6’7 and 250 lbs, Gronk is a freak of nature. Imagine him being a power forward in the NBA? He wouldn’t be as dominant, but he would be a lot like a Charles Barkley, maybe with a little more speed. In baseball, he would be a power hitting first baseman, or corner outfielder. Vladimir Guerrero perhaps?
Michael Vick: Lightning fast early in his career, and a rocket arm. In the Major Leagues, he could have played anything but catcher and probably excelled. A middle infielder or center fielder. I see him as a Barry Larkin or Lance Johnson. If he played basketball, I imagine him as a speedy point guard that could shoot, slash to the basket, or make the precision pass. Isiah Thomas in his prime?