The Lady Sleeps
What sort of villains would I want in this campaign?
Low level goblins and kobolds would be fine and in large numbers could feasibly challenge higher level groups & minions.
Orcs. Definitely an Orcs & allies section, with several tribes making inroads into the area.
Undead infestations would be challenging for the group because of the effect of auras and other other status abilities to minions. A powerful undead figure would make a good long-term villain.
In terms of challenges to their temporal power, there is room for rival organisations, other Churches, rival Guilds and so on to provide conflict and interest.
With the Slow Levelling in place, these threats can be mapped out in quite a lot of detail without too much worry that the PC’s will outstrip them in short order.