I'll list the biggest WHAT IF player ever, IMO:
He was a huge disappointment. MANY considered him to be one of the best RBs ever. I'm sure the Steelers are thankful for him b/c after the Rams drafted Phillips, they immediately traded Bettis to Pitt.
Phillips has had soooo many off the field problems. He's also the guy who missed the block on Aeneas Williams that ended Steve Young's career.
All that God-given talent, yet he is, and has been, rotting in jail.
Another WHAT IF player or two:
Bo Jackson - Could you imagine how great he would've been had he not split his time playing baseball as well? Maybe top 5 RB ever.
Mike Vick - This one is obvious. Although I was never a Vick believer, you can't lie about what he did for the Falcons his first few seasons. Had he never gotten into his many off the field troubles, there's no telling what the NFL would look like today.