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.
Such a league would be especially valuable in developing QBs and OL. That alone would make it worthwhile -- at least from a fan's standpoint. Eliminate Thursday night football (other than holiday games), and play the minor league games that night. Play maybe a 12-game schedule and allow for a few late-season call-ups, such as MLB does. Expand the rosters, say December 1, to accommodate the call-ups.
I'd let players rehab in the minor league at the team's discretion. And I would let players who are suspended for other than criminal (violent crime) reasons play in the league at whatever wage it pays. A guy like Randy Gregory, for instance, could hone his skills, stay in shape, and be in some kind of structured environment, if he chose to take advantage of the opportunity.
Minor league games would be good programming for the NFL Network, which would help justify the expense. And by allowing every NFL team to have a team, there would be added fan interest.
The caliber of play in the NFL would improve, and more players would have jobs. Seems like a win/win.
The colleges would oppose because players would have an option to play football somewhere else, but it's time big-money college football was ratcheted back a bit in any case.