Add young Russell Wilson of the Seahawks to the list of athletes who didn't make it in baseball but went on to be impact players in another sport.
Among others, former Skins/Raiders QB Jay Shroeder played minor league ball in the Blue Jays system before being a success in the NFL. Danny Ainge made it to the bigs with the Jays. He played one season there at third base, hitting .239 without power. Then went on to become a good guard on some great Celtics teams.
Of course there are the two-sport guys -- Bo Jackson, Neon Deon, and Gene Conley from back in the day. But I can't think of a player who didn't make it in the NFL or NBA but then went on to star in MLB. There may be one, but a name doesn't come to mind.
One of the world's most gifted athletes, Michael Jordan, tried baseball after NBA sainthood (admittedly at an advanced age) and couldn't do it. It's tempting to draw conclusions about how slowly and grudgingly the Grand Old Game yields up its manifold skills compared to other sports.