Matthews, Marner, Nylander, Brown - can't think of the 5th? (is there one I'm forgetting?) They have Marleau, Kadri, Komarov, Fehr and Martin I think in their bottom 6. And at the moment - the only one that belongs in the conversation with M, M, N - is Pastrnak. (although hopefully Bjork and DeBrusk work themselves into it)
I might be wrong but I think Toronto picked up and kept some o.k.ish unspectacular vets so the kids wouldn't have to do the defensive lifting.
The fifth 2nd year guy is Hyman.
And I'm not trying to say the Bruins young forwards (outside Pastrnak) is on the same level as Matthews/Marner/Nylander, just that running multiple young forwards isn't a major concern, unless they aren't producing.
Certainly the Leafs line-up structure isn't the be-all, end-all, but I think there are things about how they are structured that teams can take away from and use to their own benefit, and how they use each line to get the most out of them.
Notice they have a 4th line that is 2/3 traditional with Martin + Fehr/Moore. The outlier is Brown, a 20 goal scorer. Note that they use Brown on the 2nd PP unit, while Fehr kills penalties. Kapanen was in Brown's spot to end last year before they got Marleau.
Boston's comparable would be Acciari + Nash, with Acciari in the Martin role (minus the fighting aspect) and Nash as Fehr/Moore, the PKer. They should go find a "Brown/Kapanen from the cast of Vatrano/Debrusk/Heinen.
If you go further up the line-up, the Leafs "all-in-one" line comparable to Bergeron's with Bjork is JVR/Bozak/Marner. Two solid veteran's who can play both ways with a young offensively skilled player.
If I was Boston, I'd try to create a poor-man's Hyman-Matthews-Nylander with Kuraly-Spooner-Pastrnak. Hyman is the forechecking, dig pucks out guy on this line, Babcock says he's one of the strongest forecheckers in the league. Let that be Kuraly. Babcock gets the most out of this line by paying attention to match-ups when he can.
Finally, I'd try Belesky-Krejci-Backes as the Marleau-Kadri-Komarov, a line with veteran players, who are experienced enough to handle most hard match-ups, and offensively gifted enough to produce. Giving these guys some more hard match-ups should in theory benefit the Bergeron line from an offensive standpoint. Don't tax Marchand and Bergeron so much, considering they already kill penalties, play 1st PP duty. Why force them into all the hard match-ups when you don't have to. They'll still go up against top players, just lighten the load a bit.