I guess you lost me at Football. What would you like them to do? Baseball has the most expansive farm/feeder system of all of the sports. I guess, my thought was if they were to have a rel/pro system it would have to be with the NASL/USL teams, as they are already in place.
For players, it seems like the system they have in place works well so they can loan players to teams for seasons, if they aren't going to play or want to give them more time, but can recall them at different times as well.
Maybe that isn't how it is set up for every team at this time, but I've been told that is the first step in that direction. My buddy is on the Chicago Fire and plays D for them, but being behind Johan Kappelhof has caused him to be put out on loan the past season and a half. But he was brought up in situations throughout the year. He felt it was a great situation for him, as he thought he probably wouldn't be able to contain the Giovinco's and Depsey's of the world at this point. And he was only in St Louis so he could make the drive up to Toyota Park if they needed him to suit up.
I think a G-League type set up is a good system for the development of US soccer players, not sure how it would fare for the money-makers in the MLS though. They would have to take their lumps and promote USL/NASL teams for the first couple years until they establish the teams that will be going up and down, kinda like how the EPL has teams in the Championship that are the longest shot ever to reach the EPL, but they need to fill out that level with teams that can sustain.