Off the top of my head I came up with these masks when the Lvl. 15 wizard in my group found a shady peddler of magical wares in a dark alleyway. The vender had several low level items I knew he (the wizard) would be dissinterested in, and one he had to press for. A polished dark wooden case with tribal symbols carved into it. In the case, three wooden masks one an old man, one a young man, and a young woman. When a mask is placed on your face your face is replaced with the face on the mask, and your face is transferred to the mask. The wizard, happily paid tens of thousands of hard earned gold pieces for them. And began thinking up all the awesome things he'd be able to do with this new power. The thing is if he puts a new face on, his face will be on the mask. So while he is walking around with the face of whatever poor souls face was on the mask, anyone can who can steal his mask can steal his face. I hope I explained this well, I have a habit of rambling.
It seems like you could use a 'Magic Jar' type of effect to come up with rules for the masks. Each mask would be a different soul receptacle, the soul being represented by the face. However, The mathematical conservation of faces indicates there are 3 too many faces in the world. I would be curious if there aren't 3 faceless beings bent to track down the artifacts and retake their face, lost in their creation.
That's a good point, I had thought of that but just had the original faceless people suffocate and die with no mouth or nose. But faceless people desperate to regain their humanity at any cost would be creepy and would set up for some moralality vs. power dilemas.