I was doing some thinking about Opening the Dark mechanics. The magic system is a descendant of Ars Magica and Mage: The Whatever. An interesting rule is that every character effectively has a 0th rank (which is somehow distinct from having no rank at all) in the "Dark Art," allowing them to cast spells if they have a dark artifact to provide a positive rank. This means that even muggles can cast dark art spells if they have a dark tome or whatever.
I was thinking of expanding this concept to, say, folk magic. This would, for example, allow characters to perform minor magical actions such as protection spells without needing to go overboard on things like vulnerabilities for magical creatures. I might want some nuance where not all ghosts are inherently warded by salt but still want a character to be able to use a circle of salt for protection. Substitute "salt" for any other apotropaic such as Buddhist prayers, brick dust, true faith, etc. It's easier to place all that stuff under the umbrella of "Folk Magic" rather than trying to account for all imaginable vulnerabilities in the monster entries.