Advice on writing detective fanfiction?

[TheArcaneWeirdo]

So, I'm writing an Alanik Ray fanfic, and I've been debating on something.
One of my favourite things in detective stories is the riddle they present to the reader. When the clues are all there, and if you stop to think while reading you can stay one step ahead of the protagonists in solving the mystery.
The original Sherlock Holmes stories aren't written like that, but even Sherlock canonically agrees they should be.
I checked, and this style of writing is called 'fair play detective story' - and there are several rules for writing it.
One of the classic rules is that no supernatural agencies should be involved.
It shows how outdated the list is - it was written before the modern fantasy genre, where magic is an inherent part of the world that's expected to be part of the plot and has laws of its own that can be explained to the reader, was developed (or at least before it was well-known).
The site where I read said list (TVtropes) said in a side note that it's okay to include a fantasy element as long as it's introduced ahead of time and explained to the reader.

Fanfiction should make this even easier, because the readers are already familliar with the original story's worldbuilding and ready for it to be played with.

But what if the fanfiction is for Ravenloft - a campaign setting with a ton of setting-specific mechanics and species, that are different between editions, and also has a long tradition of 'fanon' (as if, headcanon widely accepted by the fandom) homebrew content?

Like
If the villain in my story is a certain monster, should I limit its abilities to the basic 3e stat, for simplicity's sake? Or would that be more confusing for people who expect the 'dread' version (unless the twist is that it's an outlander one)?
And what about netbook stuff? Can I expect people to recognise something that was mentioned in a random QtR article?
Where would you draw the line, for my first attempt at this?

Or maybe I should include whatever I want, and just explain stuff as if I would if it was my original worldbuilding no one is aware of yet?

Sanderson's First Law (As in, a fantasy element's importance/usefulness in the plot should be in direct correlation with how well the reader understands it) might be a good compromise, but what do you think?

[Drinnik Shoehorn]

With detective stories/whodunnits, start at the end and work back. It’s how Agatha Christie wrote.

Have your killer, motive, murder and victim all in place before you begin. And there are rules to a whodunnit;

1) the killer must be introduced in the first third of the story.

2) it must be deducible by the reader who the killer is.

3) the reader must have access to all the same clues as the detective.

4) no supernatural “gotchas”.

In Ravenloft, the last one might be hard. But if you do use magic and the supernatural, have a consistent, internal logic that the reader can follow that doesn’t cheapen the reveal.

[TheArcaneWeirdo]

4) no supernatural “gotchas”.

In Ravenloft, the last one might be hard. But if you do use magic and the supernatural, have a consistent, internal logic that the reader can follow that doesn’t cheapen the reveal.

What I'm debating on is how much of this internal logic I should bother to explain, if I'm going by the logic of the mechanics presented in an existing sourcebook - and whether the sourcebook being canon or homebrew makes a difference.