In no particular order:
Running Back - Pollard is in the last year of his contract, and Zeke has been fading for a while. A new fresh top RB prospect needs to be added to the team.
Offensive Line - both Center and Left Guard are issues. But if Tyron Smith comes back, you can shift Tyler Smith to Left Guard. But a more powerful center is needed to open holes in the run game.
Tight End - paying Dalton Schultz a top tier contract for middle level performance is not a good use of resources. The Cowboys are going to need those resources to sign Defensive players, such as Diggs and Wilson. So drafting a new starting Tight End is a need. Ferguson and/or Hendershot are serviceable journeyman type players, but a Star Tight End could be an asset.
Defensive Tackle - unless run defense improves dramatically, the Cowboys still need to improve their interior line defense.