Bruins need to send back either Krug or Backes in a Tank deal. (SEE EDIT AT BOTTOM OF POST) For it to be Krug, STL would have to value him as a positive asset and have it not just be swapping salaries. That "value" could be "worst case we can flip him for a Shattenkirk type return" or he could be someone that is part of the long term solution.
I would think DeBrusk would have to come from Boston's side for sure as the primary young piece. DeBrusk is our 22 year old Jayden Schwartz and is the big get.
Beyond that, I would think anyone aside from Pasta or McAvoy would be on the table, its just finding the right grouping for both sides.
When thinking about a specific proposal, I was drawn towards including higher character "could wear a letter on his jersey" sometime kinda guys given where STL is at right now in that department. In Boston's system (outside of the aforementioned Krug, Backes and DeBrusk who all qualify) that makes me think of Studnicka and Frederic. Both seem to have #2C ceilings, but the Bruins brass seems to be hoping one of them pulls a Krejci/Bergeron and bumps past their ceiling. I would think one of those guys would be an ask for STL, with the 20 year old Frederic maybe being the favored as the hometown boy.
I also would think Carlo could be of interest to STL if they are looking to deal one of AP or CP. Carlo doesn't have the offensive upside of those guys, but he has been playing in the top 4 of one of the top few defensive teams in hockey since he was 19 years old. He is very good and still growing and improving. He is a weird one in that he could just be what he is (a big RH #4 defensive D that can skate) or, at 22, could have another level. Really hard to put a value on him.
Other notable 22 and under guys in the Boston system include:
Vaakaneinen - LD - smooth smooth skating defensive minded two way guy that Bruins hope is McAvoy's long term partner. Played a few games for Boston this year before he got an elbow to the head. Has been out since then. I would think DeBrusk + Vaak + Krug would be too much for Boston to swallow. Near NHL ready.
Zboril - LD - been playing top pairing in AHL before he was also upper body injured on a high hit. Great skater. Nasty. Doesn't seem to have the offensive upside hoped for at the draft. Near NHL ready.
Lauzon - LD - man Bruins have a lot of LD. Less skilled than the two above, but still has 2nd pairing upside. Has good size and looked good as a complimentary D man in his time in Boston this year. Defensive leaning two way D. Near NHL ready.
Bjork - LW/RW - Another very good skater. Kind of in the "too good for AHL, not quite NHL ready" tweeter period. He is 22 and looks like a top 6 forward some shifts and then you forget he was playing. Top 6 upside and near NHL ready.
Donato - LW - Great shot. Good offensive instincts. Found scoring in the AHL easy. Has looked a step behind at the NHL level. Been a disappointment so far this year.
Heinen - LW/RW - Cerebral two way guy. Think Loui Eriksson style game. 47 points as a rookie last year. Slow start this year, but picking it up now. Not big or fast. Just smart. Can play against or with anyone.
Senyshyn - RW - He will always get some side eye in Boston because he isn't Barzal or Connor. But, after being asked to play in the bottom 6 last year in a defensive role to get rid of his cherry picking ways from Jr, he has been at a goal per game pace the last few weeks in the AHL. He is big and he is very very fast. Could make a push for the team later this year, but certainly next year.
JFK - C - Two way probably #3C with some creativity to his offensive game. Plays at a slower pace and thinks the game a lot. Playing #3C in Boston now and seems to have solidified that spot. Might have enough offense for #2C, but he will have to learn how to play with more pace as he isn't Krejci good at slowing things down.
So here are a few specific offers to choose from!
DeBrusk
Krug
Frederic
Lauzon
DeBrusk
Vaak
Backes
Frederic
DeBrusk
Carlo
Backes
Lauzon
edit: actually, if we wait until closer to the TDL, the Bruins could make the deal without moving Krug or Backes. Which kinda leaves the specific proposals above moot.