So, right now I have an unhealthy obsession with Hattori Heiji. discordiansamba on AO3 (think they're on fanfic as well) has a good selection of fic about the Osakan detective that's only augmented my favor. I haven't read through all of their fic, but I'll gladly rec what I have. discordiansamba (if you're on here) if you want me to take this down for any reason I will (I like your fic a lot U^U)
(if anyone thinks any of what I've written needs to be put under spoilers, please tell me!)
Okay, first up. Blood and Snow. Basically, it's canon divergent (some time around 2012 I think). Heiji, who is, of course, a skeptic, is forced to face the facts that there are things in the world that can't really be explained away. In other words, vampires. This fic is so good you guys. I was leery to start this--mostly bc I don't really like vampire stories all that much. But it's written so well. It's completed, with 20 chapters and an ongoing sequel, Bloodsport. Both stories have blended casefic into supernatural-esque fic so good!
Speaking of fic that does casefic/supernatural blending perfectly
(Don't) Believe What You Know does it so well. It diverges after the Nue arc. Basically, it's Conan who has to let go of his skeptical views for Heiji's sake. Because Heiji is constantly knee-deep in supernatural cases for a very good reason. Honestly, watching Cone investigate Heiji is so great and I just love this story so much and you should go read it this instant. It's up to date and ongoing.
I'm in the middle of reading Twist of Fate , which basically takes the premise of Detective Conan, and switches Conan with Heiji. So far, it's pretty great. I'm not caught up on all the chapters, but I'm liking it. It's up to date and ongoing as well.
All of the above fic have canon pairings.
And I'm reccing one last fic from this author (although honestly, there's more that look good that you should check out as well--I just haven't got there yet). By the Moon's Light is a fantasy AU. It's not Heiji-centric like the other ones I recced--more of an ensemble piece, but this author is so freaking good so go read this anyway. I'm just going to post the summary, because a) it's only 5 chapters in so I don't know where the story's completely going yet, and b ) I wouldn't be able to do it justice-- When her kingdom's royal treasure is whisked away by a mysterious thief in white, Princess Suzuki Sonoko sets out to reclaim it- and maybe capture the handsome thief's heart while she's at it. What starts out as a simple quest driven by a maiden's heart soon involves countless people, dangerous curses, ancient witches, and a long hidden truth waiting to be revealed. It's mostly canon pairings, with the exception being Sonoko/Sera (which is such a good--and too rare--pairing that I don't even care :3).
Honestly, the people in this fandom are so creative and smart:))