A one-off story that took place in a fictional universe that you think was an interesting enough setting to have potential for more stories, not necessarily sequels.
So I guess Middle Earth doesn’t count?
I will go with the world of Pushing Daisies or Edward Scissorhands. I like these worlds that are fairly anachronistic.
Philip José Farmer has been gone for a while now. He wrote a series of novels/novellas/stories about a fictional Riverworld where everyone eventually is resurrected.
I wouldn’t mind someone continuing the tale.
Firefly 
Zoe : I know something ain’t right.
Wash : Sweetie, we’re crooks. If everything were right, we’d be in jail.
Zoe : Sir, we don’t want to deal with Patience again.
Mal : Why not?
Zoe : She shot you.
Mal : Well, yeah, she did a bit.
Book : I’ve been out of the abbey two days, I’ve beaten a lawman senseless, I’ve fallen in with criminals. I watched the captain shoot the man I swore to protect. And I’m not even sure if I think he was wrong. I believe… I just… I think I’m on the wrong ship.
Inara : Maybe. Or maybe you’re exactly where you ought to be.
The OP did specify ‘one-off’ stories. So that rules out (for instance) Discworld and The Culture.
But it doesn’t rule out Good Omens, or The Algebraist.
The Fifth Element universe.
I’d like to see a return to Kumandra, the land of Raya and the Last Dragon; or Zootopia.
I’ve wished that we could get reactions from other parts of the world to the events of “The Nine Billion Names of God.”
Pushing Daisies is one of my all-time favorite shows, and I agree, part of its charm is its oversaturated, slightly retro world.
My other choice would be Chromatacia, the quirky, dystopian future England in Jasper Fforde’s novel “Shades of Grey,” in which the entire society is built around which colors a person can (or cannot) see. It was published in 2009, and while Fforde has talked about sequels (and may be working on a sequel now), it’s still, at this point, a one-off book.
When I finished Neil Gaiman’s The Ocean at the End of the Lane, I was so taken with it that I immediately reread it, the only time I’ve ever done that. I think that world was a one-off and qualifies for this thread.
Another one-off was Ursula LeGuin’s The Lathe of Heaven in which dreams change the real world.
If the OP hadn’t specified one-offs, I would have agreed with Kayaker here.
Although I’d prefer the World of Tiers. Whether using Farmers Kickaha and cronies (prior to the Farmer novels would work) or different characters in the same setting would be fun. For that matter, as several of the inhabitants of the lowest level are immortals from the Trojan war era, you could even have Riverworld-esque moments of ancient characters rekindling their desire to explore or conquer. 2 for 1!
But back to the OP, more material set in the Snow Crash Universe (Neal Stephenson) would be my choice.
I was so looking forward to even more adventures in the Dark Crystal universe. I’m glad we got what we did from Age Of Resistance, but it wasn’t enough.
I’d also like to see side-stories from the Indiana Jones series, and more stories in the Pirates of the Caribbean universe that have nothing to do with Jack Sparrow. I guess you could argue Jungle Cruise is such a film. Perhaps a new Haunted Mansion could complete that as a semi-trilogy.
You know there’s a sequel (or series 2) to the TV version in the works, right?
Great!
Well, there was The Diamond Age, set in the same “universe” except it takes place in China, not America.
Speaking of which, remember Rats and Gargoyles? Where the Earth exists in a cosmos run by Hermetic alchemy, and the story unfolds in a nameless city, extending so far it spans time zones, built by inscrutable Lovecraftian gods who have invaded our reality and warped space and time according to their fancy? Except, the human gods are still around, lurking in the sewers. It might be interesting to learn in a sequel how all that pans out, but only if it be really out there, as well as deeply meaningful re. all the philosophy thrown about, so as not to be disappointing.
Or The Carpet People, or the settings of Dark Side of the Sun and Strata. Which are my nominations.
Also, more works in the Neverwhere universe, please.
Strata crossed my mind, too. A kind of real-life simulation, so to speak.
I personally think those are more a set of aesthetics and atmospherics than actual settings, although I also like them. I think the screen versions of The Mysterious Benedict Society and A Series of Unfortunate Events have similar aesthetics and atmosphere, which is one reason why I enjoyed them. I’ve only seen Edward Scissorhands once and don’t remember it that well, but I loved Pushing Daisies. It’s definitely my headcanon that The Mysterious Benedict Society takes place in the same world.
I agree. It really felt like it was in the Pushing Daisies World. But Edward Scissorhands could also fit.
The Forgotten Planet, by Murray Leinster.
The plot: a vast human empire sends out automated ships to terraform planets. At particular stages in development, new ships are sent to progress things to the next stage. One such planet gets stuck at bugs and fungus because it falls off the grid for whatever reason and no more ships come. The story follows the descendants of a crashed space ship - because the planet is so inhospitable and deadly (all the bugs have of course grown to monstrous sizes), they’ve regressed to a primitive hunter-gatherer society.
A series about a team that goes around cataloguing, fixing, exploring, etc. failed terraforming projects would be pretty neat.