Most prospects have only one star potential - even in real baseball.
Think of it like this - most bench players in the majors are one star players. And most minor leaguers don't project to even be bench players in the majors. Even a good minor league system, like the Marlins, has according to John Sickels, one of the better people who ranks prospects, only 11 players who rank above a C+.
If your players show improvement in their abilities through their minor league performance, their potential will increase (and if they are hurt or play poorly, their potential will decrease). But most players in the minors will never have major league success, in the game or in real life.
BTW, Sickels has only nine Cleveland prospects ranked above a C+. You can check out his top 20 here:
http://www.minorleagueball.com/2008/...dians-top-20-p