It looks like it's a campaign book made to give a little bit of everything so people can expand on Ravenloft themselves (or if they're huge fans, they'll go back to older editions to fill the missing stuff).
I get why they have small entries, because do we really want huge reprints of older material? That's the issue of new editions, you're really just buying reprints (much like how video game companies sell you a "remastered" version of the same game, or the same game but instead of PS4, it's PS5, or it's part of a bundle, then part of an even bigger bundle, etc. etc).
I think if the domain entries were "This is the domain, what's it known for, etc." and then present the biggest changes to it now in comparison to previous editions, then all people have to do to flesh it out is take the old info but replace some of it with the new lore.
What's the most important thing nowadays with D&D is how do the mechanics work with the lore? I think what sets this campaign setting apart is the offering to DMs and players every game mechanic tool and ruleset along with advice on how to run these Horror-Fantasy D&D games. It should heavily offer how magic is altered in Ravenloft, the Mists, new rules for Fear/Horror/Madness, etc. Just update the old mechanics to 5th Edition (for DMs) and offer player material suited for a Ravenloft game to the PCs (like a Van Helsing type subclass, Ravenloft-themed spells, etc.).
Certain magic aspects of D&D need to be addressed for a proper Gothic Horror game. The biggest one being Detect Evil. Which I'm going to assume they'll address. But plenty of other magic need to be taken into consideration as well (Necromancy, divinations, resurrection).
