Surprisingly enough, the team is pretty set at RD right now. McAvoy and Carlo as 1RD and 2RD for the future, with Clifton now as a lock for 3RD. Andersson is in the system developing and there's still underrated Victor Berglund for depth.
LD is a bit more volatile, although they have prospects who can fill that side, of course. Lauzon, Zboril and Vaakanainen will all be battling for essentially one spot depending on how things shape up with Krug and Chara. If Chara is done after this year and Krug stays, then you're looking at one of those three prospects to fill a top-pair or bottom-pair role depending on where Grzelcyk fits into the pairings.
I don't see how all three of those kids stay with Boston and expect one to be moved this season or after, but adding another LD with potential isn't a bad thing.
And in about four-years from now, we'll be seeing Dustyn McFaul in the AHL or NHL as a two-way, second-pairing stalwart. (shoutout to
@Dom - OHL for putting him on the map last year)