OK, as promised, here are my chief alterations to the setting (that I can recall off the top of my head). Major changes are as follows:
Geographically, two domains that were just too bizarre (Bluetspur and the Nightmare Lands) were never part of the Core IMC, but always Islands. Historical events that link these locations to the Core, such as the Thaani exodus to Barovia or Dr. Illhousen's explorations, happened via the Mists, not direct overland travel. (Hence the Core wasn't overrun by the illithids hundreds of years ago.)
Daglan became an Island under Radaga instead of vanishing after FoG, and it later merged with G'henna to form a Cluster called the Withered Expanse. Radaga's motives are much more complex and nastier IMC -- honestly, did
all female darklords have to be obsessed with their own looks, at the dawn of the game-setting?

-- and she and Yagno Petrovna really,
really hate each other.
Like most DMs, I've added an Island or two of my own invention. Most distinctive of these is probably Aftermath, a
20th century domain and an homage to early Stephen King; its darklord depopulated his entire planet (!) by unleashing a lethal biological weapon in a botched attempt to murder a romantic rival, and his mind can animate the many abandoned machines throughout his domain, to communicate with intruders from his hidden germ-proof bunker (via any radio or telephone) ... and to attack those same visitors, once he convinces himself that they can't possibly be real "survivors" and he's only imagining their presence (eternal loneliness having driven him to a desperate paranoia).
Population-wise, Ravenloft's always had problems, in that too few people are present to sustain too many man-eating monsters. However, I didn't want to spoil the claustrophobic effect of the setting by making it larger (with some exceptions; Souragne
is somewhat bigger than in canon descriptions IMC). To eliminate problems -- which, by my calculations, lead to things like Jacqueline Montarri decapitating her way through fully a third of Barovia's female population in the first 50 years of her curse!

-- I made some "native" monsters of Ravenloft that are particularly voracious into Mist-napped imports -- e.g. if my PCs meet a skin-thief, it's probably only been in the setting for a matter of months -- and altered some of the individual villains so they don't
have to kill so often ... which, if anything, makes them seem more evil when they choose to do so anyway (Montarri IMC
could survive indefinitely without a head, she's just too vain to go without one for long).
If a domain is really,
really getting underpopulated, the Dark Powers aren't above slipping an extra village or two onto the map when nobody's looking, and altering the memories of all parties involved to say that it's always been there. This information is NEVER in-character knowledge; so far as PCs know, the monsters really
can wipe out whole domains, even if the DPs don't want the victim-supply to run out and the domains' cycle of misery to end.
Historically, events in the Grand Conjunction modules wrapped up a little differently. Instead of the real Material Plane, the darklords "escaped" to yet another construct the Dark Powers had established (complete with look-alikes for Strahd and Tatyana as "royals"; honestly, reincarnation is one thing, but credibility has limits for Pete's sake!

) specifically to mess with the participants. The Holy Symbol of Ravenkind took no part in the storyline --
that was around Sergei's neck, not on an alter, when his murder took place; read your own *%&#@ novels, people!

-- leaving the Icon as the PCs' objective in "From the Shadows". The Book of Keeping wasn't retrieved from the Material Plane, but on a
second visit to the past, this time to the crypts under Castle Ravenloft. Nathan Timothy was also fortuitously killed by the heroes during this adventure, explaining why Arkandale got absorbed into Verbrek.
I
really don't like the concept of "personality fragments", nor the notion that simply
wishing to escape a horrible fate can create one -- if it could, wouldn't Ravenloft have more fragments than 'complete' people? -- so IMC the Tatyana who died in Jander Sunstar's arms really
was the original one, albeit rendered ageless by having ingested Strahd's blood when she bit his lip to get him off her (a la the Kargatane secret society). All "Tatyanas" we've seen in the centuries since then are fakes: the DPs find it amusing to play off how Strahd's professed "love" for her is entirely superficial, as these copies'
personalities are actually overidealized charicatures, not accurate portrayals of Tatyana's nature (she had more depth). It's also their smart-arse way of giving Strahd the very thing -- "someone like Tatyana";
like her, yet certainly not actually
her -- which Sergei, himself, had wished for his brother moments before his murderer struck.
Likewise, the man who'd actually built the Apparatus wasn't a calved-off fragment of Strahd von Zarovich, IMC, but a Mordentish "local" named Erik Leede. When Azalin's escape-ritual tried to separate Strahd from his bond to the Land, it formed a link with the Good-from-Evil extractive effect of the Apparatus, and this rebounded to disintegrate Leede, leaving only drifting whisps of his essence (a la VRA's "alchemical philosophy") and no one to be split in two. The Apparatus then sucked Strahd and Azalin through this planar cross-link, and it split the
vampire in two. Strahd's resulting "Good" self -- really the inoffensive, albeit introverted scholar he would have become, had he never gone off to war in the first place! -- could not cope with the recollection of having been a vampirish villain, but when he inhaled the lingering essence of Leede which drifted through the lab, he became fixated on Leede's memories and convinced that
that had to be his true history, not the terrible vampire he also remembered. The "Evil" Strahd (still a vampire, hence unbreathing) was never exposed to Leede's gasified essence, so it took "the Creature" a while to figure out what had happened and to build up his forces; in the meantime, the "Good" Strahd became convinced his (real) memories of vampirism were the result of some awful darkness within him, so moved to Mordentshire (under the Von Zarovich name) and began living the life of an alchemist (rather, "the" Alchemist).
Madame Eva is far more of an active (albeit behind-the-scenes) "player" IMC, in that she's been secretively fighting a low-intensity 'cold war' with the Dark Powers for centuries, having spurned their attempt to make her Ravenloft's second darklord back in the 5th century (when they still bothered to ASK villains to accept their "heart's desire"

). The DPs wanted to exploit her unprecedented psychic gifts, to predict the outcome of their endeavors -- they weren't as powerful back then; IMC, they've been getting stronger over time, which accounts for why the Land of Mists' growth has been accelerating -- but her Chaotic contrariness led her to set them up, by dropping hints to Jander Sunstar which would let the elf win free of the DPs' manipulations (which he
did: he perished in the sunlight as he'd intended, back in 499 BC [and the fact he was such an old vampire made no difference BTW, as aging only makes vampires resist sunlight IMC if they age
in Ravenloft]). Eva's severance from time, her murder in 495 BC, and the Zarovans' sterility (no kids since Mikhail, b. 492), all date from the DPs' realization that she'd reneged on their "arrangement"; however, the raunie had already set several events in motion -- some of which my own PCs have contibuted to, though Eva's schemes for revenge won't bear fruit for quite a while -- which might well undermine the Dark Powers' intended goals (!) ... that is, if even
Eva's Sight has shown her
everything she'd needed to learn, to make this happen.
The ToUD prophecy is just that: a prophecy, not a metaplot device. No actual date is alleged, for the events it foretells. This means that some believe it'll happen countless generations in the future, some believe it's coming next Tuesday, and a heck of a lot believe it's a load of hogwash.
Lyssa von Zarovich didn't become more powerful via a ghost's aging effect

, but by spending an extended period amongst the Arak, back when they still lived below the surface. Lyssa has a number of sith allies (who'd found her intriguing as the first vampire their race ever met) back in the Shadow Rift, and could probably get help from them in a pinch if she could find a way to enter that domain; her later visit to Bluetspur, in fact, was something she set out on, because her
first attempt to ally with an utterly-nonhuman race from a domain all mortals dreaded had been so productive!
The Malocchio-as-Dukkar subplot has been downplayed. When I
do get around to bringing it up, I'm probably going to decide that Malocchio really
can't remove darklords from their domains -- if a true fiend can't do it, and a full-blooded Vistani can't do it, why should a hybrid of fiend and quarter-Vistana be able to?

-- but that he's been fostering that rumor (and maybe even setting up
Azalin himself to believe it; how's
that for a reversal?

) in hopes somebody who wants out of the Land of Mists will engineer his
own escape from Invidia!
As for Castle Ravenloft, I've eliminated the witches and re-stocked that room with Sergei's original bedroom, complete with MASSIVE ethereal resonance and phantoms which re-enact his murder. OTOH, instead of the spirit of Sergei himself, I put a twist to the "House of Strahd" notion that PCs can ask the ghost in his crypt for help: while a ghost that
looks like Sergei is indeed present, it's actually
Alek Gwilym's spirit, impersonating Sergei so he can rat out his former commander and killer, Strahd, to such heroes as might avenge their deaths! Unlike Sergei's, Alek's spirit is not Good-aligned, and is perfectly willing to send would-be vampire slayers to their deaths, if there's the slightest chance of getting back at the comrade who'd betrayed their long years of friendship. When he's not masquerading as Sergei for intruders, Alek hides from Strahd in the one crypt the vampire shuns like sunlight ... and takes petty pleasure in using his
Ghost Writing salient ability to manifest dreadful puns and insults to the Von Zaroviches on the lesser tombs within the castle's vault.
The Dark Powers IMC, as correctly surmised in my sig, are indeed complete and total bastards. Their actual
motives aren't something I can reveal on this board ... but I will say that, from what I find when I look at their actual
behavior in those few cases where their direct or covert intervention is acknowledged, they're not always as stolidly mature as is generally believed ("Lord Through Dark"/"Dark Lord Through"/"Through Dark Lord"? What are we, five years old here...?

). Not to say that
moral maturity is necessarily equated with chronological development, mind you....
