My definition of a great player who never was in the National Football League is going to include a whole bunch of players.
It's going to include the draft busts. The guys who had all the talent in the world but also had major attitude problems. The players who never had the proper coaching (perhaps the same as a draft bust). And it's going to include the players who suffered injuries that derailed or ended their career.
Basically this list includes any player who significantly underachieved in his NFL career, and yes, you can be a fantastic NFL player and still be on this list—as long as it's easy to say that you could have and should have been so much more.
This is basically any player whose career wasn't what it could have been.