It's very simple. I've said it for years - this team has been lacking a true PMD and a true #1 center. McD appears to have evolved into one of the best all-around dmen in the league, which most certainly puts a dent in that first problem, although if we could add an offensive force (perfectly willing to sacrifice some defensive acumen) on the right side as well, that would be a godsend.
Most important, however, is to find a guy to play the middle who can dictate play. Win faceoffs, be close to a PPG (doesn't have to exceed or even equal in this day and age, but you need a guy who at least flirts with 75 points annually), win physical battles, win speed battles, carve up the defense.
Stepan is a below par #1/above par #2, who has the brains and the hands, but is lacking the physical and is held back by his lack of footspeed. Brassard IMO has evolved into a par #2 who has the speed and the hands, but is also lacking the physical and doesn't have Stepan's hockey IQ. Richards at this point is a below par #3 in my book - he has the brains, but is just simply shot.
If Miller can evolve into a Mike Richards, that would certainly be helpful, but that still doesn't quite do it. What we truly desperately need is an up-and-coming Barkov/Galchenyuk/Johansen/Schiefele.