I don't know about other sports in the rest of the world, but in the US all the pro sports have unions representing the players who collectively bargain with the club/team owners for standardized terms and conditions. There is no world governing body that the NBA, MLB, NHL, or NFL listen to, so they handle their own contracts. MLS follows most of FIFA rules but still handles their own collective bargaining (there was a major dispute in the US about solidarity payments to lower clubs where MLS had to move closer to FIFA rules). I know LigaMX also has their own closed system that royally fucks over players in that league. So instead of FIFA doing all the bargaining and controlling the transfers of players, the PL would have a standard set of terms and conditions for contracts, which would be different from the BuLi and different from L1 and LaLiga. Maybe the BuLi contracts have game limitations for players but the PL doesn't? Or maybe L1 guarantees a percentage of transfer fees to players?

While I like to think this would give some power back to the players, since they could start having preferences on which league they go to based on their contract conditions, we all know the mega-clubs will find a way to fuck over players. This might also lead to collusion among billionaires exactly like what happens in LigaMX.