I think the thing that would help the NFL the most would be to create a minor league system like MLB has, though only one minor league, not several like MLB uses. That would allow players of all ages the ability to play on teams coached by NFL minor league coaches and show they can perform at the NFL level. That would mean less time wasted on many top round draft picks who end up as busts. Instead, most drafted players would go into the minor league first, and then get promoted to their NFL team. Now, some players could still go straight from the draft to the NFL, but this way, teams would be filled with more players who can play, rather than holding spots for players not ready to play but would be claimed if not protected on the active roster.
The NFL needs to stop using college football as their minor league because college football does not play by the same rules or even have the same play style as NFL teams. Even college teams that run so-called "pro-style" offenses are not running true NFL offensive schemes.
Rather than some 8-team NFL Europe type of league, the NFL should copy MLB and allow every team to create one minor league team. They could do things differently such as once a player has played 2 years for a minor league team, another NFL team can make an offer to sign-him and his current NFL team could either match the offer, promote him or let him go.