ALL OF THEM, throw out the entire NFL rule book and make the NCAA rule book the new NFL rule book.
The reason the NCAA rules work is because of the difference in ability. If you did change the rules in a league where the ability is more equal, QBs would never complete a pass...ever. Corners would beat the crap out of receivers all the way down the field and then screen them when the pass was coming in. Then the pass rush would obliterate million dollar QBs because they have no one open to throw to. I can bet that owners would never want their investments being obliterated on a regular basis.