As much as i love Rantanen he's not a 55 assist guy. Odds are that'll come down a bit, MacKinnon probably isn't going to be beasting like he was last year to the same extent and that'll affect both players. So basically that duo will probably see some regression in terms of PPG.
Danton Heinen had ridiculous production in the 1st half last season considering he came out of nowhere to the big crowd at least (31 pts in 37 games til Jan 13th) and he's probably not going to play top 6 for the Bruins.
Also as good as Barzal is i feel like a lot went right for him offensively last year, maybe he'll come down a bit, wouldn't be the first time for a 2nd year player after a very successful first season.
Those are the guys that come to mind, but honestly, not a whole lot guys who i see coming down that much, and even the ones i mentioned could very well keep it up.