Other than police procedurals (and there are I’m sure subgenres of those, but though I go on occasional police-procedural binges — now — it’s not my main playground, and I don’t normally examine them that closely) there are a ton of mysteries that fall somewhere in a gray area.
They all follow the general mystery structure of opening with a crime, having the first phase of inquiry, after which you have the second murder (often killing your main suspect) then a series of interviews (I highly recommend those in which the interviews take the form of fights or other unusual means of acquiring information) and finally the denouement and restoring the world to its proper place, with the additional chapter to provide the reader with a “cigarette moment” being optional (but appreciated by this reader.)
But there are variations that influence that structure and I’ll mention some variations.
Take Rex Stout’s Nero Wolf mysteries. Are they cozies or not? They fail on the front of not being PRECISELY as mannered as British cozies, but OTOH they are certainly not the blood and guts type realistic mystery.
What it actually is is a “relationship mystery.” The relationship in question is friendship between two men, and in this it follows the pattern or Agatha Christie’s Poirot mysteries. Only in this case it is more of a Sherlock/Watson thing, in that one of them is the brain and the other the “gatherer of information/forager in the world” Or in this case, Archie Godwin is the muscle to Nero Wolfe’s brain.
So why do I call it a relationship mystery? Because it is. Often the precipitating incident, reason to investigate the crime, whatever originates in the two main character’s relationship, which in this case is often one of getting on each other’s nerves.
I’ll note a lot of these, these days, are not friendships but romances, (hetero, usually) and that this isn’t a bad thing. If you create a compelling enough relationship over several books, I have found I will read them even if the mystery is meh, because I want to know what you’re doing with these interesting characters. I suspect I’m not the only one. Note though that you can’t give them happy ever after on the first book for this to work. Often the relationships that will have me binge a series are well night impossible by reasons of either personality or society.
Place/time/society mystery. There was some years ago a woman doing Hollywood mysteries. She might still be. I started reading electronic and lost track of her. The structure is still the basic mystery structure. HOWEVER the place/time/type of society is an extra character. Assume that your readers are reading you because they were attracted by the location or peculiarity of the setting. You need to make sure your interrogation scenes/etc. exploit that setting to its full extent, or you’re going to piss of the reader. For extra dollop of fun make sure the motive is something relating only to that setting. (Yeah, Hollywood motives can be hilarious. Or bizarre. or both.)
Craft mystery – I think I already mentioned this. It is not enough for your character to meet the murderer/witness the murder at a craft show. For this to work, your character has to have special knowledge derived from the craft that enable the solving of the murder. This is why I couldn’t do a “real” craft mystery. While I do crochet, I don’t go to shows, and I never learned the lingo. I just do what I see.
Supernatural mystery – instead of interviews, etc, you’re allowed to have dreams or visions or whatever.
Then there is the hard boiled mystery, which is NOT a procedural. These often billed themselves as realistic. They’re not, of course. They’re just mysteries for the people who want to be shocked by the descriptions/motives/etc of the crime and therefore think themselves tougher than those who read cozies.
These mysteries often use shock value/blood and gore to distract you from the obvious solution.
Since the ethos of the writing is “we’re all damned” often there is no punishment for the crime.
Next up: the peculiar structure of police procedurals.