Auston Matthews, USA
Matthews is a very clear lock and was already named to the roster, for good reason. At this point, he is a clear top-five player in the NHL and is projected to retire as one of the games most prolific goal scorers.
He is a perennial Rocket Richard candidate and consistently gets votes for the Hart and Selke trophy. He will likely be their top-line center and quite possibly their best player as long as he can recover from injury in time.
Mitch Marner, Canada
Marner is one of the NHL's premier wingers and although he was not one of the six players already named to the roster, he is as close to a lock as can be.
The resume speaks for itself, he is a player who is consistently on pace for 90+ points while also being one of the best defensive wingers in hockey. Although he lacks the hardware, if he can continue scoring at the rate he is, he could very well end up in the Hockey Hall of Fame one day.
William Nylander, Sweden
Much like the other two star Leafs' players above, Nylander is a lock to be a member of team Sweden already being named to the roster. This honor is very much deserved and despite not getting as much recognition earlier in his career, he has produced 80-points or above each of the previous three seasons and shows no signs of slowing down so far.
His play somehow gets even better on the international stage, having played 21 games for Sweden at the World Championships, scoring 15 and 37 points. He currently projects to finish his career as one of the best Swedish NHL players of all time and will undoubtedly go down as one of the best Leafs' players ever.