Café de Nuit

This idea is one I think might be fun to have develop over the course of a campaign, using a little of the elements found in VRGTR

The idea is that the party is pursuing who will eventually be their BBEG, but at the moment, are just a run-of-the-mill evil target that adventurers usually find themselves crushing. They are potential darklord material, but the potential is just a seed of it at the moment, but enough to get the passing interest of the Dark Powers. They pursue the enemy into the mist, and they come out in a small parcel of land. It's just enough to hold a single building and some land. Likely an inn, a tavern, a manor, the remains of a keep, something which the characters can use as a home base. There they meet a helpful NPC, likely a Vistani, who welcomes them to the Land of Mist and a newly discovered pocket of land in it.

The party, using knowledge of how to travel or with the Vistani's help, pursue their target, discovering that they have been traveling through different domains and getting connections, making deals, creating a network for an overarching goal of becoming powerful and perhaps even conquering the new lands they find themselves in, depending on the enemy the DM has dreamed up. Along the way, they grow more powerful as they reap the fruits of their exploits and fail more Dark Power checks.

As the campaign progresses, as the party levels up, the mists recede from their little home base, exposing more and more land around it. By level 5, it is the same size as expansive grounds, by level 10, it's large enough for a settlement. By level 15, it's a small duchy, by level 20, it's a country. As the mist recedes, they meet people who somehow say it's always been there and some of its history.

At the end of the campaign, the party has to face their BBEG in this newly discovered land, because the last part of the BBEG's goal is there, but perhaps by then the party has figured out the true nature of Ravenloft. Perhaps they haven't. They face the BBEG and try to bring them down. The fight is epic for a final clash, and depending on the actions of the party, several results could happen:

1. The BBEG has been allowed to roam free long enough that they have gotten enough collected corruption and on their defeat, the Dark Powers intervene, making that BBEG a Darklord of the domain they PCs have turned into their home, forever locking them in a stalemate against the BBEG they have been pursuing for so long.

2. The Party kills the BBEG, but none of the party have gotten corruption of their own, or have undertaken quests to redeem themselves. Upon his death, no one in the new Domain is evil enough to be a Darklord candidate, so the mists swallow the land and the party. The party either find themselves back where they began before the mist, or find themselves still trapped in Ravenloft, heroes perhaps, but a glimmer of light against it's eternal darkness.

3. The party defeat the BBEG, but one member of the party has also gained enough corruption and upon defeating their rival end up taking the mantle of Darklord from the BBEG and claiming the land as their own. Their fellow party members are horrified and either must flee, or be forced to try and stop their once comrade in their evil. Or perhaps the other party members have also collected corruption and become the new Darklord's minions instead.

While I generally like the idea, just a word of warning:
Level 20 is a looooong road and Ravenloft is not really the setting to be higher than heroic tier; i.e. I believe campaigns should end at around level 9-10, before you get at paragon tier.

The "guy that was supposed to be crushed becomes darklord" is a theme that resonates well in Ravenloft. A guy that blames the failure of his "Most Evil Plan@" on everyone but himself (lackeys, gods, PCs, chance) etc. There are a few darklords similar to that, I know there are... but none comes to mind.
Inferiority complex leads to megalomania.

Interesting idea, the concept of a freshly formed domain waiting for a darklord to be chosen from a fleeing villain and the dark anti-hero pursuing him was one explored in . The seed idea is also a little bit similar to what happens with a rod of security, in that it becomes a potential domain waiting for a new darklord. However, to my knowledge there has never been anything exploring a domain arising from a simple building to an entire country.

As mentioned above, it might be a good idea to truncate the campaign, especially if the end domain is a single country rather than a continent or a planet (which are perhaps more fitting arenas for 20th level characters.) 5 levels is a long time to play in a single complex.

You might also give thought to having multiple nemesis/darklord candidates that the players latch onto but yet never manage to defeat. For example, an interesting campaign variant might have the domain begin as an intentionally created trap orignally fabricated by mortal hands (like the rod of security) but not necessarily the one the characters were originally pursuing. The entry of the PCs and their villain into the situation could draw ire from the rod's original crafter as well as crafter's originally intended target(s), any of which could end up as the final BBEG depending on how developments with the PCs go.

Hadnd't considered level caps in Ravenloft. Part of my interpretation for higher levels is some magic is nerfed, like Plane Shift and such which the Dark Powers don't allow to work like normal. Or they have consequences of using, like Necromancy which risks corruption. Getting that high in level would risk open conflict with Darklords depending on their attitudes like Drakov or challenging Azalin or Strahd. That would turn it from horror to more dark fantasy admittedly.

The seed domain, as my initial idea, would be mistaken as part of another domain at first, like a plant's leaf which will eventually be clipped to be transplanted. Characters would navigate the mist to enter the main domain, but the mists receding would make such a trip more and more perilous and/or difficult as the mist got thicker and mist horrors have a greater chance of appearing, hinting more and more that the area is a domain that is slowly developing with potential darklords.

The multiple potential darklords is a good idea. In their pursuits, the party unintentionally cuts down their competition which makes it easier for any of them to become Darklords themselves if they continue to be reckless.

So, perhaps an alternate idea to the Seed of a Domain. The campaign takes place in a cluster of seeds, each one with a potential darklord in them, each inside one aspect of the land. A potential evil, each scheming for their own ends, their own ambitions, attempting to reach their own goal. For each one the party brings down, that territory is added to the seed domain they are based in, unintentionally allowing the BBEG they are chasing to become more powerful, having more resources, and more capable of achieving their goals.

The idea of the mists receding and leaving an expanded domain is also somewhat explored in the Mithras Court novel.