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I thought of a way to move toward football players playing both ways. Allow separate defensive and offensive player, limit substiturion of those offensive and defensive players. This wouldn't do as much to tackle the weight problem, but it would do quite a bit. Many teams now have run blocking specialists and pass block specialists on their offensive lines, pass rushing specialists and passing rushing and run defensive specialists. If those type players were required to stay on the field for entire offensive or defensive series, the type of player who succeeded would change over time.
This method also has the advantage that it would not put anyone out of work immediately, so the players' union is less likely to stop implementation.