Chris Jones would be the dream pick up, but I don't see him leaving KC. Nor do I think the Cowboys would even make the effort to sign him even if they could.
That being said, give me Justin Madubuike DT Ravens. Young, disruptive, comes from a great organization, tough minded defense and may not have peaked yet as a player.
I would go after him and Queen from the Ravens. This defense needs to get tougher as a whole and more stout in the middle of the field. The mindset the Ravens players play with is contagious. Their culture is much better as an organization than Dallas, but hopefully adding the right people can change the culture in Dallas even slightly.
I know all of this is a pipe dream because of the front office approach to free agency and the cap. But, these players plus re-signing Stephon Gilmore and adding impact players in the draft would be ideal.