Sam Bennett
Teuvo Teravainen
Andrew Copp
Ivan Barbashev
JT Miller
A few I can think of. Though I think there are always variables that won't line up. Some guys just get better once they receive more ice time or better linemates, some through a change of scenery.
It's a loaded question hopefully not to prove anything.
Any late bloomers can be picked apart as not real comps - not the same stats, age, position, usage, talents, better/worse team...
Then if you can't find the perfect comp, which you won't, the narrative that nobody has ever been a late bloomer centerman top line skilled with 2.03 p/60... gets repeated. I'm hoping that's not the point.
No I'm genuinely curious because I think Mittelstadt becoming a regular contributor is a long shot. Typically late bloomers will fill another role (Copp/Couterier come to mind) successfully and offense comes later. So they are still contributing even if they aren't racking up points, so they continue to get ice time while that part of their game develops.
I think Mittelstadt will be a pretty rare case if he does work out. An undersized skill guy who isn't producing won't continue to get a roster spot after so long. The only real comp that seems like a real parallel is Josh Bailey. So I guess we'll have to wait and see.