If anyone has watched the show Black Sails, I believe the first episode shows a scene where a crew prepares before battle by putting facepaint on and one character wearing sharp and pointed false teeth. This kind of intimidation was common place in piracy I'm sure and might be possible for the Sea of Thieves.
Adding a new cosmetic type that covers the players face could give the players a little more customisation on their pirates as well as the chance to strike fear into their enemies. I can imagine a new player being absolutely terrified of a half naked pirate wearing a skeleton skull on their face.
The possibilities for this kind of feature are only limited by the dev's, and our imagination;
-An ancient tribal mask (think aku aku from Crash Bandicoot)
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The reapers mask as a standalone
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Skeleton masks
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Maybe even some novelty ones like the iconic Comedy and Tragedy mask associated with the performing arts.
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Simple face coverings like bandana, a balaclava could be cool but wouldn't work with the time period, though I may be mistaken.