I believe that the best thing the Cowboys defense can do to improve is stay healthy.
The defense took incredible steps last season in the right direction thanks mostly to their defensive coordinator combined with an aggressive draft aimed at rebuilding the defense. They also stumbled upon the best defensive player, most likely, in this decade.
That's nothing compared to where they might have been if they had been able to stay healthy. That defensive line was riddled with injuries to the point where the projected starters, Lawrence, Gregory, Gallimore, and Hill collectively started 22 out of 68 possible starts. That is less than a third. There was not a single game in which they all started. There were five games in which three of them started. That was the tail end of the season, they never had the opportunity to become cohesive.
There were times in a game when the entire defensive line was composed of rookies, Parsons, Golston, Osa, and Bohanna. As unfortunate as that was, it will pay dividends in the future as all four of them received valuable experience.
There was also the demise of Jaylon Smith whose previous injury resulted in an erosion of his skillset. Losing Jabril Cox early also hurt them. They were also without the services of a veteran free agent that was supposed to help against the run, Bret Urban. As a result of losing the majority of their inside defenders the run defense suffered and was exposed in the playoff.
Today, the defensive line looks as though it hasn't been as strong since the 90's in both talent and depth. They are three deep in DT's. The group of Osa, Hill, Gallimore, Bohanna, and Ridgeway should make the defense one of the strongest against the run....provided they stay healthy.
There is other good news. It is believed that players make their greatest improvement in their second season so last year's draft class should contribute even more than last season, at least most of them. I'm not as inspired by the 2nd year DB's as much as I am with the D-line.
There is the continuity of the same defensive philosophy since they were able to retain Quinn. That is huge.
Now, when we can add in a few exciting defensive rookies like Williams and Bland as well as some quality veteran free agents like Fowler and Barr, I think we can look forward to a new Doomsday Defense emerging in the NFL. The only thing i see preventing that from happening is another injury pandemic like we have seen too often over the past several seasons.