Ravenloft's "Other Domains"

 Van Richten's Guide to Ravenloft has fairly in-depth discussions of many of its domains, from Barovia to Valachan, with pages dedicated to the personality of its Darklord, a section about its key locations, and usually a thing that's unique about that domain that you can build adventures around, like stages for Falkovnia's frequent zombie sieges or a renown system for dealing with the rival factions in Kalakeri.

However, the book details several other domains listed that I think work fairly well as "elevator pitches," with one or sometimes two paragraphs giving you the basic gist of them. Some of these are long-established domains while others are brand new.

I thought I'd mention some of my favorites among these:

Cyre 1313, The Mourning Rail:

I love the idea of a train dungeon - for one thing, I've always loved having more modern technology encroach (or suffuse) fantasy settings (I went crazy for Final Fantasy VII Remake, which if anything is near-future). The premise here is that on the world of Eberron, there was a lightning rail train set to leave the capital of Cyre, taking would-be survivors out of the country before the disastrous Mourning left everyone dead. But the Darklord, its "Last Passenger" was some high-up person (presumably a member of a Dragonmarked House) who insisted on kicking some of the passengers off in order to maintain their privacy. Not only did this doom the people kicked off the train, but it also delayed the train long enough that it never escaped the Mourning. So while the passengers are desperately hoping the train can go fast enough to escape the impending disaster, none realize that they're all dead already.

Vhage Agency:

This domain is just a single private detective's office in which everything is in black & white. Its Darklord runs the detective agency, and sends adventurers out the door and into the Mists to help her solve cases, though she hides the fact that she's a murderous criminal who is involved in every case she investigates. I love the idea of making this domain a kind of hub/base of operations for the party, and again, it ventures outside of the classic fantasy timeframe. (I love the idea that the party can get food here by ordering the fantasy equivalent of Chinese food, which gets handed to them by a wraith-like arm jutting out of the Mists out the front door).

Tovag:

This classic realm is the home of Kas, as in the Sword of Kas. Vecna's vampiric former lieutenant was trapped in a kind of shared domain with his master after the two had a falling out way back in 2nd Edition, and that edition's final adventure modules series was a campaign that involved Vecna's escape from Ravenloft. But it seems Kas was left behind and has also never figured out that his nemesis isn't there anymore, so he constantly sends troops into the Mists to attack Vecna's stronghold, and none ever return.

Odaire:

Ok: evil Pinnochio, a village taken over by evil toys who killed all the adults. That's the basic pitch.

The Rider's Bridge:

A no-muss, no-fuss domain that's perfect if you just want your party to fight a Dullahan. It seems to connect to other domains periodically, but you could easily have this just be a weird one-shot jaunt into Ravneloft.