Long term and short term moves that set them up for a legit run this year and clear space for next.
Heinen, B level prospect (Fitzgerald, Cehlarik. etc) and a pick (dependent on how far they go in the playoffs) for Krieder, adds a fast, gritty scorer to the top 9. Moore to Arizona for Lane Pederson. Clears 5.5 for next year if Kreider is not resigned, at this point assume Backes is bought out which would clear another 2M Beleskey and Siedenberg clear another 3M. That would leave about 25M for next years FA's DeBrusk, Krug, Chara, Lauzon, Grzelcyk, Bjork and Kuhlman. Biggest cap hit will be Krug then DeBrusk the rest should not be huge contracts leaving money to sign Kreider.
Either way your playoff roster would be
Marchand-Bergeron-Pastrnak
Kreider-Krejci-Kuhlman
DeBrusk-Coyle-Bjork
Frederic/Blidh-Kuraly-Wagner
McAvoy-Chara
Carlo-Krug
Lauzon/Miller/Grzelcyk/Clifton
None of the RW's past Pastrnak are great but you have good pairs on the top three lines and Kuhlman and Bjork should be able to compliment the other two playing simple games and bringing speed. I would like some more size on the RW but it's a tougher forward group with Kreider, Frederic and Blidh and the D has good size that can be mixed with speed when needed. Putting Lauzon and Miller as your third pair helps vs Washington, Grzelcyk and Miller work vs Tampa or Pittsburgh.
A good coach will be able to mix and match, sadly the Bruins have Cassidy so whatever moves they make may be moot with him still running the bench.