It went from a full campaign setting which it was in 2nd Edition, and I believe 3x, back to a demiplane - and the plot hooks tie it directly to the Realms - making it Realm-centric. In 2nd Edition, the only campaign setting cross over that was blatant was Lord Soth with DragonLance (both Hickman/Weis properties).
But as I said, I'm not trying to piss on anyone's cornflakes. I honestly don't think any setting should get a source book, including my favorites. Instead they should get UA material expansions. The old settings have such a glut of material that, honestly, having their limited staff (my understanding is that Hasbro is less then generous with the HR stick) and materials focused on publishing - for the 100th time - highly generic top level fluff for FR, DL, DS, PS, My, RL, GH or anything else is a waste of time. Take the APs and be happy.
Seriously, if you were a RL fan how would you feel knowing your favorite setting just got torpedoed to provide a gothic horror element to FR?
Well, I still disagree with your assessment of adding a gothic horror element to FR. The only plot hooks I see are the "How to get into Ravenloft ones" and nothing else Realms-specific continues. All I see is here's how to get from the Official Campaign Setting to this new campaign setting.
Having said that, there was a 3rd and 3.5 editions of the campaign setting, licensed to White Wolf. They removed Lord Soth, Vecna and Kas, and renamed Bane if I recall, all of which came from other settings. I can't remember whether they were in the original Ravenloft CS box, or added later, to hook into the other major campaign settings.
I also seem to recall that after the license agreement ended that WotC planned a 4e one but it was never released. From my understanding, the 4e Dark Sun campaign was also well regarded. The licensed approach could be another option again, too.
Personally, I think that since CoS has done well, I wouldn't be surprised to see a campaign setting for Ravenloft. The reason being that they aren't releasing regular material as they are for the Realms in the APs and other products. It would make sense to me to release an AP like CoS to see if the market is there, then release a Campaign Setting, or at least a partial setting.
Incidentally, if it was to test the waters for other campaign setting material, it (along with Planescape (are the obvious first choices, since they are easy for characters in existing campaigns to access. Eberron is more difficult, as is Dark Sun, which really works best with characters made specifically for that campaign with their flavor and unique take on the standard races and classes (and then some)
The one I think is probably farthest down the list is Greyhawk. Since it's no longer the default for "generic" D&D, it's not different enough from the current Realms to stand out. I certainly wouldn't mind it, but it would need the right authors and approach. I think Eberron, Dark Sun, Planescape, and Ravenloft are the most likely to get a larger campaign setting treatment.
They've been releasing a lot of class expansion UA articles recently. I could see a hybrid product that is half PHB-style new rules, combined with a new campaign setting.
Another option would be to follow in Volo's footsteps and release a new take on Van Richten's guides, which were among my favorite supplements ever along with Ed Greenwood's original Volo's guides.
Still not sure that I'd need a FRCS anymore, although I'd happily buy it. It's the only setting I use at this point, and has always been my primary campaign since it was first released.