I don't think it will happen, but it would be nice if the Blues could free up cap space by dealing Shattenkirk for a cost-controlled young forward, and perhaps by shipping out a couple extra warm bodies.
If that occurs, I would like to see them flip Berglund for Hartnell. Given Hartnell's contract, a mid-range pick or prospect would likely need to be involved also.
Short term, I think that improves both teams. Blues replace some of the grit and leadership they lost by not resigning Backes and Brouwer, and CBJ get a defensively responsible center/wing who will improve their puck possession metrics immediately. Hartnell's age and Berglund's inconsistency are a wash. I think it's pretty even value.
Long term, the deal is less appealing for the Blues--depending on Hartnell's health. But I'm assuming they're still wanting to compete this year, and taking on a bad contract that helps us do that isn't necessarily a bad tradeoff. I think it's worth the risk.