For labyrinth fanfiction that goes deep on world-building and actually includes maps, I've had the most luck on Archive of Our Own. It has a pretty robust tagging system, so you can filter for the 'Labyrinth (1986)' fandom and then add tags like 'Worldbuilding', 'Alternate Universe', or even 'Maps' sometimes. Writers who are invested in expanding the Goblin Kingdom or sketching out new sectors of the Labyrinth itself often use those tags to attract readers who crave that extra layer of detail. The 'Alternate Universe - Worldbuilding' tag is a particular goldmine for stories that reimagine the rules of the fantasy setting beyond what we saw in the film.
Another solid approach is to look for authors who are known for their lengthy, epic-style fics. These multi-chapter stories frequently come with author's notes that include links to external art or maps they've commissioned or drawn themselves. Tumblr still functions as a hub for a lot of this supplementary material; an author might post a detailed map on their Tumblr blog and link it from their AO3 story. I've found some incredible hand-drawn layouts of the Labyrinth's heart, complete with notes on magical ley lines or the territories of different goblin clans, just by following those links.
If you're open to older platforms, some real treasures are buried in dedicated Labyrinth fanfiction archives like The Goblin Market or Labyrinthine. These older sites might not have the slick tagging of AO3, but the stories there often come from a time when authors would embed maps directly into the HTML of the chapter pages. The navigation can be a bit clunky, but the payoff is discovering narratives that treat the Labyrinth not just as a backdrop but as a living, breathing character with a full geography to explore. I still revisit one story where the author provided a different map at the start of each story arc, showing how the paths shifted with the protagonist's changing mindset.
Finally, don't underestimate the power of a direct ask in a community space. The Labyrinth fanfiction scene on Discord servers or specific subreddits can be wonderfully responsive. Posting a request for 'fic with extensive world-building and maps' often yields personalized recommendations from other fans who have exactly what you're looking for bookmarked. Someone once pointed me to a crossover fusion with 'The Dark Tower' that had such meticulously crafted cartography I spent as much time studying the drawn hallways as I did reading the text itself.