Taking into account Davis's writing structure, traditionally, the villain is someone that's been shown once or twice as either a new character or someone harmless. My money isn't on either the Yukimura parents; she's been targeted one too many times and while parents can absolutely be persuaded to kill their kid, it's the exception, not the rule. Between the two of them, I'd go for her father being responsible over her mother, for no reason other than it takes an extraordinary person to live with someone that powerful and not figure out ways to get a piece of the action.
Another "new" presence is that deputy. How he'd tie into the narrative, I have no idea. But he was a deliberate inclusion in the plot. And there's that woman who wanted to know about the She-Wolf and her cohorts, but I think that's something else.
If we're going to go off-map slightly and say "okay, maybe it's someone we already know", we already know Deucalion's still around and actively monitoring Beacon Hills. Don't think Gerard Argent has the contacts still in town for it. And um There were two deaths that might have awakened the Nogitsune's need for pain, anger, vengeance - the first was Paige Rivera, but the timing's off. There was another murder the night of your sacrifice for your parents - that of the Darach. Th So you should probably be aware that's on the table.
And it's possible that it's just working alone. Its fixation on Lydia isn't good. Banshees are powerful impetuses for all sorts of magic, so we may have a bigger problem on our hands pretty quickly regardless of the Nogitsune's origins.