I think a minor league with each NFL team having a 2nd team, would work very well. Standard would be higher than college football - its full time football and even rookie free agents who just about get an invite to a NFL camp are among the top percentile of college football players.
Good way to develop players and keep vets in the game. If I were setting up the minor league, I'd allow the recruitment of any player over 18. 3-4 years in a pro set up run by the NFL will produce a much better player than any college football program.