There have been quite a few NFL rival leagues since the 1920 season. All came and went. Two were successful in having some of their member clubs merge into the established league. But most just bled money and went quietly into the night.
There have only been two official farm system leagues that the NFL had financial and player ties to. The Continental Football League operated from 1965-1969 with a format of three NFL clubs sending players to a single team as their official farm club. NFL Europe was a huge success that had teams in European cities to introduce the game of American Football to those countries plus was a farm system set up to give the bottom third players more experience and also to incorporate free agents.