I'm all for it. More football, more players, more coaches, more scouts. NFL teams have coaching internship programs now, this could be an extension of that. I also agree with Booze that rosters should be expanded but would add that the number of gameday or "active" players should increase. The owners may not like paying more players but coaches would love it and the NFLPA would sign off on more jobs. I think this helps protects club better against injuries as well.
Maybe they could do an eight-team league, most likely in the south (SEC country), and each team has rights to developmental players from 4 NFL teams. Teams that are not traditional or geographical rivalries. For example, a team in Orlando would get players from the Cowboys, Vikings, Dolphins, and Bengals. The Head Coach should be a neutral hire, like a Dan Reeves or a Steve Mariucci, but the staff would be made up of intern coaches from the 4 assigned NFL teams. Play 7 games and then have a championship game between the top 2 teams. Would definitely help kill some time in the Spring.