Your comments re: Barzal are all fair of course (though I disagree on the analogy to Nash...more on that later) and like I said I certainly wouldn't be disappointed if he were to make the team, but I just don't like how he fits on the team. Its an awkward fit imho. His strongest skills are his speed (plays well at any position/role), puck carrying, and puck distribution. The latter two skills play better at centre, and Canada has typically built from the centre out at the best-on-best level, especially during the Yzerman/Armstrong/Babcock era.
Assuming Barzal would be playing the wing position, like he does on Long Island (and due to him not exactly being a Selke candidate when it comes to defensive play), he'd be a puck carrying/distributing winger lined up next to either McDavid, MacKinnon, or Crosby (or perhaps Point). Those skills are redundant given the elite talent at centre, particularly with Marner slotted-in on the right side presumably ahead of Barzal that more or less offers the same playmaking skills as Barzal (with less speed, but much better defensive play). Those three/four centres need complementary skills on their wings...wingers that can chase down loose pucks, win puck battles in the corner or along the boards, finish scoring opportunities, create space, provide defensive cover when needed, and maybe provide some sandpaper and make life difficult for their opponents. Aside from offering up his blazing speed, Barzal isn't a great goal scorer and doesn't seem like the type of player that can fill in gaps on any of those top lines. At centre, he wouldn't be strong enough defensively or on the faceoff dot to really carry a line, imo.
In my mind, Barzal's best opportunity at making the team would be to out-compete Marner for a playmaking wing position, or out-compete someone in Thomas/Suzuki/Horvat group (likely Thomas as the frontrunner) for a #4C role. Maybe there is a role for him as a 13/14th forward that can come off the bench if someone gets hurt or we need an injection of speed/energy. Like I said, I wouldn't be disappointed or really even all that surprised if he were to make the eventual team, I just wouldn't have him on my roster.
With respect to Nash, he was a strong skater, and a big bodied natural goal-scoring winger that has previously played in the Olympics and a handful of World Championships. He fit Yzerman/Babcock's template perfectly. Size, speed and goal-scoring ability on the wings. The better analogy for Barzal, imho, is Duchene, who offers a similar skillset and C/W versatility as does Barzal; Duchene was a better goal scorer, his speed played really well on international ice, and he was excellent on faceoffs. The same speed requirement isn't really there on the smaller ice surface.