Essentially miniature villages (ie, cities of the dead), would simply be a collection of graves hemmed in by a fence. Would spawn 1 entity of a random undead type per approximately 3 graves (identical rates as village size limits). Graves would consist of 1 gravestone and 2 blocks of coarse dirt. Small ones could generate as part of a village, larger ones could spawn as separate structures with crypt "dungeons" that spawn the additional undead mob types.
ghost (flying slime of the harmless small variety, essentially; has a chance of appearing when non-undead entities are killed)
geist (baby zombie sized mob that generates a cloud of debris around it)
wraith (cross between a zombie and an endermen, is attracted to experience orbs and players, siphons off xp with each hit, can kill you if xp goes to 0 or below)