Not recognizing 15 doesn't necessarily mean she has never seen The Doctor, as many other stories demonstrate, but not recognizing the TARDIS does seem to imply that.
Maybe she's one of the Pantheon that are being built up as this season's villains. I can't see her as the big bad for the whole season (who would presumably be The One Who Waits), but I could see her playing a part in a major late-season twist.
Or, maybe she's a Time Lord who somehow escaped genocide. Maybe one who left Gallifrey even before the Doctor did.