There is enough material in Desert of Desolation to make a full book on it's own (adding a level 1-4 prequel for a start). And it's only really Pharaoh that fits a horror theme. The last episode is more like Labyrinth with it's phoenix-infested tesseract.
I thought of The Forgotten Temple of Tharizdun, but it has two problems: it isn't very good and it isn't very scary.
A couple of the actual Ravenloft modules might be worth bringing back from the dead. It's own mummy outing - Touch of Death - spawned a computer game adaptation (Stone Prophet) so it must have had something going for it. The only other one I owned was Ship of Horrors, and since I can't remember anything apart from the ship map I guess it wasn't very memorable.
You can add horror elements easily enough, throw in some undead encounters, maybe some Jackleweres, ect...