Innistrad on its own is a little thin (to be fair, all the M:TG settings so far have been). It has one race (human), four provinces, and most of its monsters are variants of MM monsters (vampires, golems/skaab, zombies, ghosts/giest, werewolves). There isn't a lot of "new" elements like Ravnica or Theros had; at best a few subclasses and monster variants, but nothing like the expansion of piety or guilds.
Ravenloft, otoh, is a mediocre stew of ideas each sectioned off as a little "adventure" zone. For every truly interesting domain or Dark Lord, you have either a very stock/one note one OR a domain that's "its X campaign world, but EVIL" (glares at Sithicus). The truly great domains with interesting hooks probably count less than a dozen, and they're not all part of the core.
If it were me, I'd add a small note on Barovia (for more info: see Curse of Strahd), Mordent (ghost/seaside), Lamordia (golem/Frankenstein), Verbrek (werewolves), Hal-akir (mummy/ancient Egypt), Darkon (lich/dark fantasy) and Richtumount (wererats/urban). Six domains shouldn't take up much space, considering they once fit 30 of them in a hardback and had room to reprint part of the PHB in it...
Innistrad plus six domains of dread, tailored to do a full campaign (Innistrad) or a "night" of terror (pick a domain). Everything else (monsters, subclasses, etc) can be shared between them.
Call it Realms of Horror and get it to me tomorrow.