There's nothing wrong with adding to a game's canon if you're doing it to enhance the storyline. Yeah, we never saw anything north of Hyrule Castle, but that doesn't mean that there isn't anything there. Hyrule doesn't have to exist in the middle of a vacuum in space. After all, in canon, we see that it is connected in some way to Termina (even if she pretends it never happened; also, it's totally Purgatory). There can be other stuff outside of just what we see in the games.

That being said, no one reads fanfiction to hear about your shitty original Mary Sue (unless said reader is also a demented Suethor). In my experience, people read fanfiction to see the characters they couldn't get enough of in new and exciting situations. If there really was a new Dark Lord with a cadre of Nazgul at his disposal, you'd better believe Zelda would Sheik up and go right alongside Link. Or, more likely, Zelda would let Link adventure alone and run the war from the homefront (and that isn't just because I ship him with Malon). The sages would be helping too. You think just because they have to live in an alternate pocket dimension that renders them effectively immortal that they don't care about Hyrule?

The problem with this Suethor isn't that she added her own original content to Hyrule. The problem with this Suethor is that she's edged out all the canon characters and given their screentime to Jenna Sue here. Original content featuring canon characters is great, especially since with Ganondorf dead, there's not a lot for our heroes to do in Hyrule anymore.

Also, she has griffins drinking tea. The mechanics of that are mind-boggling.