Here's a bouquet of Nordic crime fanfiction found on Archive of Our Own, grouped by fandom. The tags are as found on AO3:
Anna PihlForbrydelsen | The KillingLåt den rätte komma in | Let the Right One In (2008)Millennium Trilogy - All Media TypesRejseholdet | Unit OneReykjavík Murder Mysteries - Arnaldur IndriðasonVarg Veum - Gunnar StaalesenWallander - All Media TypesØrnen: En krimi-odyssé | The Eagle If you know of any more, or want to recommend or comment on a special fanfic, please feel at home in the comment field. Or make a rec post yourself! We'd love to see it. :)
ETA: I have invite codes to AO3 for anyone interested in setting up an account. Just ask for it, I don't bite (much). ;)