Well one thing I was going to bring in was the fear, horror and madness mechanics from Ravenloft. Basically the undead are horrific to behold. Constructs made of corpses in the midlands for example. Or just a zombie shambling toward you for the first time can cause panic. Undead aren't hard to kill, they don't think to defend themselves in most cases, they just do what they are told by the dark powers that govern them.
EDIT: Also, how would you suggest making the campaign more Ravenloft-y?
If it were me...
1.) I'd start by making the world look like a regular medieval world, until the sun goes down. Castles and cathedrals, villages and peasants, knights and lords, etc. That way, the contrast between day/safety and night/terror is emphasized, which enhances the fear.
2.) Most of the world is human(ish), follow one general monotheistic faith (with different sects, cults, and other smaller forms of worship scattered throughout) and generally, insular and a bit xenophobic (since traveler often bring troubles with them). This works well with a low-magic/fantasy setting in general, and allows you to liberally borrow from lots of sources freely.
3.) I'd down play the other races significantly (most know elves and dwarves once existed, but few have ever seen them) and exotic races are practically nonexistent, and usually feared when met. Its up to you which races you want to include; I like keeping the four basic races known but the others unknown. Likewise, few evil humanoid races exist; no marauding orcs or savage gnolls; but smaller monstrous races hide in the woods (like goblins) ready to eat a hapless wanderer.
4.) I'd do the opposite and play up animals (both mundane and giant) and beasts as daytime encounters. But at night, undead, monstrosities, ghosts and fiends stalk the land. The obvious choices for gothic monsters are there (vampires, werewolves, mummies, golems, etc) but lots of other monsters work well too (shapechangers, fey, hags, sahuagin, mind-flayers, liches, powerful fiends, etc). As said, I'd avoid the "band of humanoids terrorizing the countryside" motif, but I would likewise ignore giants, dragons, and other outsiders as well.
5.) Magic is believed in, but the magic typically thought of is "low magic"; alchemy, palmistry, folk medicine, etc. charms to bring luck, wealth, love, and health. Real magic (High magic; PHB stuff) is rare and has connotations with devils, spirits, and evil witches.
6.) Generally, a good Ravenloft plot plays on issues of Trust (who is being honest with you), Illusion (are things really what they seem), Isolation, Fear, Betrayal, and ultimately Hope. Don't forget the latter; the PCs need something to fight for lest they become monsters themselves. A world without hope of making things better is a world which brings out of the worst in people, especially PCs.
That's how I'd do Ravenloft, especially low magic Ravenloft. Feel free to add as much or as little as you want. Good gaming!
