Before I go further, I would like to note that my experience with Ravenloft was primarily in a homebrew version of the setting that drew heavily from both S&S's 3E materials and 2E modules and sourcebooks, and built upon a timeline that had been established by two previous Ravenloft campaigns the same DM had run. In the Ravenloft I know, Nova Vassa was adjusting to the fact that an ocean had suddenly appeared beyond it, Gundarak fell and was divided up between Barovia and Invidia, a renegade psionic vampire therapist named Dr. Dominiani had his own island Domain, and the Shadow Rift was a great gaping hole in the center of the setting that had apparently eaten a couple of domains (Vecna's and the one from Dark Sun, unless I am mistaken) sometime within the last five years. Then our PCs came in and changed even more of it...
I say all of this to demonstrate how Ravenloft can survive massive upheavals without losing the fundamentals of the setting. So long as Strahd resides in Barovia, the Vistani wander the land, and the Dark Powers seek to corrupt and destroy those who champion goodness, and an oppressive aura of evil and dread pervades the setting's tone, then the core of what makes the setting interesting remains.
Ravenloft is uniquely suited to dealing with the emergeance of new races and classes. Adding the Tieflings and Dragonborn to the setting should be easy. We already know that in the default P.o.L. setting that the Tiefling empire of Bael Turath gained most of their power to pacts with infernal powers, and that the Dragonborn of Arkhosia were their mortal enemies. Craft a story of how a Tiefling or maybe even one of the Dragonborn who opposed them made a fiendish bargain or committed a sin so terrible that it would warrant their imprisonment in the Demiplane of Dread. Lo and behold, a new domain appears, Bael Turath, in all of it's fiendish glory. Now we have a whole nation of Tieflings populating a dark empire, and the dragonborn crusaders, who of course got taken along for the ride, because it just wouldn't be your personal hell without the people you hate the most being there. As the inhabitants of this domain become aware of their new geographic location, the Dragonborn and Tiefling alike will realize that they're not in the same world anymore and start to explore the other Domains. Tieflings will seek the favor of the Dark Powers, even as the Dragonborn will continue their noble quest to stamp out evil, naively working against the very nature of the plane itself.
As for the classes, Warlock seems like a thematic fit, Warlord works perfectly for Falkovnian Talons and the armies that keep them at bay, and paladins of alignments other than LG will bolster the forces of Strahd, Vlad Drakov, Azlan Rex, and the Lawgiver Church as knights in their sinister service, while letting Halla in on Ezra and the Morninglord's crusading fun. In the aforementioned campaign, I played a CG Fighter/Holy Liberator in the service of Ezra and I fought against Blackguards working for Strahd, so I don't think it really matters that we are calling all "alignment-knights" paladins now instead of making fighters take prestige classes to represent those who aren't LG.
The only game mechanic that I think really needs to exist for Ravenloft to "feel" right is the Horror Check. Unfortunately, I think that this mechanic will be the first to go in 4e. Having your character run away, throw up, go mad, or die of a heart attack because of a failed roll really seems to contradict the "everything must be fun for the player" mentality of 4e. Without a tangible mechanical effect of fear, Ravenloft has a hard time of reinforcing the tenants of the horror genre, which takes away the element that really makes Ravenloft special. Then again, maybe the monsters will actually be scary enough to warrant running away from in 4e, and this won't be a problem.
Robert "Ask Not If Ravenloft Can Survive 4e, Ask If Your Players Will Survive Ravenloft 4e" Ranting
