I think a successful fic, wrt a reader not knowing canon, doesn't rely on knowledge of canon for enjoyment. I can only speak for how I structure my own stories, but I write all my AUs as if they are original novels, so no canon knowledge is required to enjoy the story or relationships between the characters. There are always little easter eggs that canon-savvy readers might smile at, but they don't distract from the story, and someone who doesn't know canon won't be scratching their head wondering why X was a thing or why Y happened. Obviously longer fics work better for this, as shorter fics tend to rely more heavily on canon knowledge. That's not to say a shorter fic can't be enjoyed by readers unfamiliar with the canon, but longer fics seem to have a better shot at hooking readers in and making them care about the characters.