How can we create parity in all of the major European leagues?

This question really depends on what kind of parity you mean. If we wanted to create parity in skill, the best way to do that would be to cap salaries for players and trade skilled players to clubs with less skilled players. That would create clubs that were more equal in terms of skill but it might take away some of the excitement of the game.

If you want to create parity in the amount of money that each player is paid, you would need to change the system so that clubs could not buy players so easily. You could just make a rule that says that each player needs to be paid the same amount, but that might change the nature of the competition.