As the title says, I'd like to suggest making all mask items occupying the face slot of our character, instead of the head as it is right now.
The reasons are various. One would be that it would simply be the most natural thing because a mask in real life, as well as in fiction, occupies the face of a person or character, which allows them to wear all sorts of items on the head (from caps, hats and hoodies, to crowns, helmets and other headgear), while still being able to wear a mask at the same time. An another is that considering how things are right now, the mask items overlap either entirely or partially with all the items that are meant to be for the face slot (such as tears, candy, other food or glasses), which basically makes the mask occupying two of our character's item slots, instead of one, unnecessarily consuming too much space that way. With this suggestion implemented, those slots would be used to a better potential, as we would be able to wear stuff on the head, while still being able to wear a mask at the same time. I believe it's needless to say that unlike some face items which naturally and logically completely overlap with masks (such as tears), headgear doesn't . Not to mention, making this suggestion reality would also open up more room for many different outfits.
EDIT: Masks that are being worn on the head (which obviously don't cover the face), should be an exception and remain headgear (such as the ladder tour/staff reward mask for example)