Remathilis
Legend
On the other hand, Ravenloft has a lot of baggage Innistrad does not. Do Darklords overshadow most other horrors making most do domains one-note affairs. Most realms are too isolated and suffer from "why is Dark Ages Romainia next to 1700's France" type of worldbuilding. There is a number of "Classic D&D world but spooky" domains (Sithicus being the classic example) and there are a lot of just badly done domains or Lords (Van Karkov, I'm looking at you). And that doesn't even touch some of the problematic elements like the handling of gypsy stereotypes or the slave/plantation domain of Souragne. Yikes.Almost every key element of Innistrad can be found in Ravenloft except Eldrazi and Angel worship, although Ravenloft does have Bluetspur, a domain whose Darklord is an Illithid Elder Brain.
Both planes have Werewolves check, Fiends check, ghosts check, Vampires check, flesh golems like things check, creepy constructs like dolls/gargoyles check, Evil Plants and Bugs check, monsterous elementals check, murderous insane humans check.
Innistrad Eldrazi monsters.
But Ravenloft has Doppelgangers, Illithids, Drow, Duergar, Death Knights, Liches, Mummies, Illithid Vampires, Demilich, Flame Skulls, Jackleweres, for example. And most importantly Darklords.
Darklord - 1d4chan
For Innistrad you really already have all the rules you need to play it except Eldrazi, it has almost no distinct monster types unlike other MtG settings, no unique playable races, no unique classes or subclasses thanks to Monster Hunter being released already, I mean you could basically just use the art book for Innistrad or the Planeshift article. Maybe you could use a few key NPC stats or an intro adventure. That is about it.
Ravenloft on the other hand has alot more unique rules and variety to it, it's a lot bigger and more diverse then Innistrad.
Innistrad, being a simpler setting, feels more like a single story rather than an anthology of separate ones. Of course, it has its failings as well, the biggest being a humanocentric world will everything else being a monster. It wasn't designed to handle kitchen-sink D&D and as such some of it doesn't fit well.
The way I'd do it is to make a Worlds of Horror book that is if half-devoted to Innistrad and half to the best domains of Ravenloft (devoid of the core/metasetting, like CoS is) with material applicable to both. Either that, or a massive revising of the settings to fix some of its larger meta issues.