Talking to a Redskins fan earlier and he was telling me how great Derrius Guice had looked and said he hoped his injury didn't sabotage his career like it did Ki Jana Carter. That being a name I remembered from the 90's, but hadn't heard in a while, I did a quick google search and came accross this article
Ki-Jana Carter opinions are probably wrong
I remember him being very good, but I was in high school at the time, played sports more than I followed it, and it was before the emergence of the internet and all, so I didn't realize how good/highly regarded he was. Based on some quotes from some respected guys, it sounds as though Carter could've have been a great one.
A few years ago, ESPN did their 30 for 30 series and one of them was The Best that Never Was, and it profiled former Oklahoma RB Marcus Dupree. Highly touted back who left college early for the USFL but who never lived up to potential because of a gruesome leg injury.
Give me some of your "Best who never was" guys.....