1) Shift worldbuilding efforts from the Dark Lords' pasts to the setting's present
One of the issues with the original presentation of Ravenloft as a setting was that the bulk of the worldbuilding details resided in the lengthy backstories attached to the Dark Lords of its various domains. This poses several problems: it puts the emphasis on powerful NPCs your players may never interact with, it creates convoluted histories that are difficult to deploy in actual play, and it robs the color from the lands and peoples of Ravenloft.
The solution here is fairly simple: focus worldbuilding attention on detailing opportunities for adventure in the Dark Lords' domains rather than on the tragedy of the Dark Lords themselves. Of course, the tragic failings of the Dark Lords should color their domains, but making them the central (and often singular) object of interest is a mistake.
For example consider the insane backstory that comes with Baron Urik von Kharkov, Dark Lord of Valachan: Kharkov was originally a panther, who was later polymorphed into a man, who fell in love with a woman, but was then returned to panther form, in which he killed the woman he loved, polymorphed back into a man, and later became a vampire. That is too much cruft surrounding a basic idea and strong theme.
Instead, simplify here and make Kharkov a werepanther who killed the woman he loved--that's the core bit that resonates. From that thematic idea, build outward: make the element of barely constrained bestial nature the keynote motif of Valachan. Make outbreaks of animalistic violence a recurring image in the domain, add in adventure ideas inspired by Jacques Tourneur's Cat People and its remake, play up the violence of cannibalism and how it is effecting the domain right now, etc.
In place of overly complicated backstory novellas like Kharkov's biography, I'd do a global revision of Ravenloft by:
- Adding factions. There aren't nearly enough interesting scheming groups in the setting; a Gothic setting, in particular, calls for cults, secret societies, orders of vampire hunters, etc. Some interesting secret societies were added after the original boxed set--such as the ultra-Freudian Dark Delvers, a group of cavers who believe that the "Hated Mother" resides somewhere in the subterranean depths--but nothing was really ever done with them. This is a missed opportunity.
- Add religion. The first presentations of Ravenloft were silent on the topic of religion because that was TSR policy at the time due to the Satanic Panic and a desire for mainstream marketing. Today? That doesn't matter. Add in a Catholic-inspired church for maximum Gothic-ness. Use the already extant lore on St. Markov as a starting point.
- Schemes > backstory. We shouldn't abandon the Dark Lord concept altogether. In fact, we can go a long way toward making it a highlight of the setting if we make what the Dark Lords are up to right now more interesting than their tragic pasts. What are their vendettas, what are the plotting, who are their allies, and who are their enemies? That's the gameable stuff.