I think the most profound addition will be a year of experience for Guyton and Beebe. I believe that both will be greatly improved, technically.
I believe that a year of NFL conditioning will have made then stronger with increased stamina.
I think that they will contribute to a more cohesive unit, more used to the guys around them and with a better understanding of what their teammates ae going to do under certain circumstances. This should make them more efficient.
When we are talking about improvement in this way from the O-line, it has a "trickle-down" effect of improving every other aspect of the team, particularly the running game.
If the Cowboys had selected a RB during the first two days of the draft then I might have said the greatest improvement was the quality of the new RBs. It might still be but we won't know until the season is underway.
However, as it stands today, I think the greatest addition to the team is last year's experience obtained by our young O-linemen.
And really, I think they improved every aspect of the running game from multiple, competitive RB's to drafting Booker to their new O-line coach. We will see.