I'm getting more optimistic that we could see O'Reilly, Hall, and Giroux again at the Worlds this year.
First, I somewhere read O'Reilly's interview lately and he said that it would be an honor to play for Canada at the World Cup, that it's an honor to play for any canadian team. Clearly it seems to me that he doesn't think anything is given to him, so even if he is named in the first roster, I believe he will go to the Worlds, but if not, consider him being 100% there if healthy.
Giroux has already represented Canada twice, in 2013 and 2015 - despite being omitted from the Sochi team, he answered the call. If he's named into the March roster I don't expect him to join TC and neither do I expect HC even call him in that case. But if he's not selected, which might happen, then there is no reason to believe he wouldn't want to prove himself, very likely as the #1 center of the team.
I believe there's at least a 50% chance Hall is not on the first roster. In that case, he is in a similar position as Giroux that he's still yet to represent Canada on a big stage, I don't believe he wouldn't use the chance to prove himself.
Those three forwards are honestly the only ones I can think of of being somewhat questionable when projecting the roster.
Defence looks really interesting. I am absolutely convinced that, if they're available, Gio, Brodie, Rielly, Barrie, and Myers will answer the call. If Montreal is out, which l still find hard to believe really, but if that happens, I'm confident that Subban will go, which would obviously be huge. He's just the type of a person I don't believe would say no. I simply don't believe it. But mainly, I have these two reasons to belive that: First, in 2013, after his Canadiens were eliminated, he jumped on the plane and flew all the way to Sweden, even when it was just for one game. Second, he still has something to prove, he was the 7th/8th guy in Sochi, and even if he's now a better overall player and in a stronger position, I strongly believe that he will use the chance to impress Hockey Canada for what it's worth.
If I'm not wrong, the World Championship starts usually when the 1st round of the playoffs is already finished, so I would definitely wait and see what happens. There is a chance we could be getting Ekblad, Schwartz, Tavares, or Marchand - in the worst case.