Now all I can think of is my mother telling a mini-me “Don’t talk to strangers, even when they say they have puppies in their van! NOR when they say they have sweets. Do. Not. Talk. Or. Go. With. Them!” over and over again.
And Hell to the fucking yes! I am all for!
Aw man imagines how stressful it would be for the detective to have to hunt/run from people he’s grown up with her entire life!! Who knows who could be supernatural! Their sweet old lady neighbor who used to bake them cookies or the weird but friendly couple they use to hang out with in college!!
On the plus side I doubt all of them would go after the detective for their blood. They might not want or care for it, or they might like the detective enough to not bother. But yeah, I can foresee it being a bit of problem.
I would like (and also suspect), as @Mewsly said, that one might encounter shapeshifters later on.
Since demons have already been mentioned in the story, we’re bound to encounter them somewhere along the line- be it for the better or worse. 
But y’know. There’s been all this talk about unicorns being real, and you know what? You’re forgetting their cousins: the alicorn and the pegasus (…does centaurs count?)
But then, if they actually exist- and want the detective’s blood, how would they go about it? Would they stand outside their house and neigh during the night? Would they somehow manage to get into the policeforce in Wayhaven, under the impression that they’re ordinary horses? Would they actually be aggressive and try to break into the detective’s home? 
You!! You Douglas’ fans! How come none of you suspected pure and innocent Douglas? He’s totally a unicorn!!!
Sera did say there are supernaturals living in Wayhaven long before the MC was even born so was it the mayor? His wife? Who had the unicorn genes? Maybe that’s why we never see any of them?
Other theory, connected to the previous one: Now that we’ve established that Douglas has unicorn blood running through him and that his father or mother has to be a unicorn, what if they’re behind the group Murphy was part of and that N left.
Now i’ll go sleep because as you can already tell i’m severely sleep deprived. Good night from me!
What if there would be @Anathema’s witch (let’s call them Witch). Who we discover somewhere in the following books, though she has known about the detective from the very beginning and has kept a spying eye on them over the years. And the detective discovers Witch quite by accident; when they are on their way to another murder scene, this time in the woods, but catch a glimpse of cloth/hair/glimmer in eyes from the flashlight/sound of cracking wood, and therefore find Witch spying on them from the bushes.
At first, Witch is a suspect, since they are found close to the murder scene. But once the real murderer is caught, Witch is off the hook.
From then on we see Witch every now and then. We can expect them to be around whenever we have to go to the forest. For they are still spying on the detective, because of reasons unknown (that is other than the scent). Though they no longer stay quiet, instead Witch follows from a small distance and keeps asking questions about the current case or the detective’s personal life. Always disappearing into the shadows of the forest when the detective returns to the city. And when it has been exceptionally long since the detective has been to the forest we can even expect Witch to appear in some shadowy place in the city to make a snarky comment or two.
During these moments we learn a bit about Witch as well, for example, that they have lived for centuries in the surrounding woods of Wayhaven, that Witch likes to feed their ravens whilst they are eating themselves (to create a feeling of equality in the family), that Witch doesn’t like black cats because they once had one and it never wanted to sit on her lap.
In other words, it is a side character with its many quirks. Because all their spying doesn’t keep them busy enough; Witch pranks the city as well, leaving fake evidence around the place or just making people who wander through the woods confused, sad and miserable. etcetera.
And, no matter from which way you look at it, Witch and the detective do bond over the books.
Thus at the end of book seven Witch confesses they know of a spell that can somewhat cloak the super special scent of the detective. Therefore successfully securing a happy closure for the detective and their love interest and the rest of Unit Bravo.
Though Witch would not do this without getting a favour in return, of course. They would utter something like 'I will help you, but…: ‘I want to swap one of my eyes with one of yours, because your eyesight is better than mine’, or ‘you have to bring me an offer every year on this certain date and place’ OR… ‘I will help you, but you have to promise to come by more often. I quite like the company.’ Because, in secret, Witch is feeling lonesome from time to time and has grown fond of the detective over the years, in Witch’s own peculiar way.
Would that be cool? I think it would be cool. And it is kind of your and Anathema’s idea/theory/wish combined. Since it is a supernatural/witch who is not good nor bad, instead Witch is dancing all over in the grey palette.
My PoOr DOUGLAS IS NOT A VILE UNICORN DFGHJUIOP
Maybe the doctor is a supernatural or the chieftain? Since we only see the doctor once or only hear about the chief and neither seemed to be overly bothered or concerned with whats going on. And y’know? What did the doc mean when he tells us it isn’t easy seeing your first body is it when he tells us there haven’t been any murders since his (great?) grandfathers time? I’m sure a small town doctor wouldn’t see any bodies that would have to do with funerals??? HMMMM Unless I’m looking way too into it.
THE DOCTOR IS A UNICORN
Unicorns are the purest of beings! There is nothing vile about them…
They sparkle… and are secretly ponies who have Narwhal horns pasted on their foreheads
i will say this… if there have been supernaturals in Wayhaven for a while, then it makes sense that A) Rebecca knew the detective’s condition because I’m guessing that one may have tried to use something on the detective when they were younger and nothing happened and B) there is an Agency outpost nearby a little sleepy town.
Perhaps there’s a LOT more supernaturals than we think in our quirky little town.
Sera’s on tumblr trying to kill me 
Suavewell strikes again. Add to that Adam being a grump, Farah singing badly, and Morgan getting bounced out of the bar… 
How can we not love these vampires? It would be impossible not to love them
now lets listen to these songs I have never heard about
Now that we know Supernaturals have been in Wayhaven for awhile which one of them do you guys think was appointed as the MC’s spy? I’m sure Rebecca left someone in town to keep an eye on us - I can’t really blame her, what with the rarity of the MCs blood.
gasp what if Tina is Real deep undercover spy?
I’m suspicious of Verda, honestly, I think he is some kind of Supernatural.
He’s 1) The only one to comment on MC’s hand wound besides UB. 2) He only moved in recently. 3) He is very handsome and seems to be immune to cold. 4) He is one of the few characters the game asks us how we feel about point blank besides Bobby.
That would be such an interesting turn. What do you think he would be if he was indeed a supernatural?
I’d guess an incubus given how many thirsty fans he has.
I don’t think Verda’s a supernatural because if he was, why would he comment on the blood? Why draw attention to it? The detective finding out about the supernatural would put him at risk were he one.
Also, from Sera’s Tumblr:
A would probably grit their teeth, get out of bed, go grab a hot water bottle, get back into bed, shove the water bottle over the MC’s feet and try to go back to sleep and ignore the smile they can’t help but give when the MC whispers, ‘Thank you’.
Cute. But what about socks, lol.
M would probably tense up immensely, turn around, nuzzle into the MC’s neck and whisper, ‘Do that again and I’m pushing you out of the goddam bed’.
You know they love the MC because they give them a warning first.
Speaking of Verda being hot, I have an MC who i head-cannoned having a slight crush on him, and for some reason when Verda first met UB I imagined my MC like
Okay but raise your hand if your character takes that as a challenge.
That’s the one thing that makes me unsure 
Maybe hes just much more concerned with doing his job well?