Some people put too much emphasis on position. The truth is, every forward on the Blues roster not named Tarasenko or Reaves is, in reality, a utility forward--you can plug them in anywhere on the roster and they'll perform fine, chemistry issues notwithstanding.
So, when gauging the Blues' needs, it's not a simple matter of determining what positions they are weak in--it's a matter of identifying skill sets, abilities, characteristics, etc., that will help them ice a more balanced roster.
As the team is currently configured, the Blues' biggest areas of weakness are:
-Speed--someone to lead the forecheck, break open plays, and keep opposing defenses honest. Overall lack of team speed has been a glaring problem Hitch has been harping on for years, but you can only squeeze so much lemon juice out of a lemon.
-Size--Blues finally addressed that on defense with the emergence of Parayko and Edmundson--which was sheer luck. Blues right now no longer have the personnel to play a "heavy" game--and unless they mitigate that by adding more speed and skill, it will be a problem. Promoting Jaskin, who hasn't shown that he can use his size effectively at the NHL level, isn't going to help.
-Grit--With Backes and Brouwer gone, we need someone with big brass ones capable of parking in the crease and deflecting shots. Right now, Ryan Reaves is the only guy on the roster who fits the bill, and that doesn't inspire much confidence. Also, if other teams take liberties with our young skilled players, who is going to step up to defend their teammates? Like it or not, that stuff does happen. We don't want a guy like Fabbri or Schwartz having to moonlight as Billy Jack--they deserve to have at least one teammate in the top-6 rotation who can step up when necessary.
-Leadership. Like him or not, Stastny is not a leader. I don't see Steen as being much of a leader, either--he only has a year on his contract, and he's a sulky, introverted kind of guy who just isn't ideal for the role--though he certainly leads by example through his work ethic and admirable consistency. Tarasenko just isn't ready. Pietrangelo will almost certainly be the next captain, but he needs more veterans to help shoulder the burden.
Playmaking--And of course, like any team, they can always use an upgrade on playmaking, "finishing" (i.e., goal scoring), and such. While players can swap around the lineup, you still need to have a pivot who can make plays, win faceoffs and be responsible in all zones.
-Trading Shattenkirk for Kreider would address size and speed.
-Trading Shattenkirk for Bjugstad would address size and grit.
-Trading Shattenkirk + Lehtera for Stepan would address playmaking and leadership.
-Trading Berglund for Hartnell would address grit and leadership.
Certainly there are other options. And there are solid arguments against trading or trading for any of these players. I'm just suggesting these are the areas the Blues are most likely looking at, and I'm sure they are exploring ways to address them within the salary cap constraints they have.