As far as being part of the canon, it depends on what Aonuma says. Four Swords/Adventures feature distinctly different styles from most of the mainline titles, but were both incorporated into the timeline, so the style of gameplay really doesn't have any bearing. From what I've seen of Hyrule Warriors though, it looks to be an amalgation of existing universes over its own. I'm personally of the mind that it's about as canonical as Link's appearance in Soul Calibur II. It can fit into the existing universe, but you'll never see it mentioned in a mainline title. Essentially like a filler arc in manga-based anime.
Of course...the fanbase might disagree anyway, regardless of if Aonuma were to include HW in the timeline. Some fans only treat the console games as 'mainline', despite both Windwaker and A Link to the Past having sequels released on handheld systems.
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