From #710:

I believe it would be nice also if you could set a "default-npc-skin", where the player's skin is by default with no value (because there might be a player's name set to player or null or none, idk), that way you could set a default skin and all npc citizens would be created with a default skin, wouldn't still that work on offline servers? the citizen shopkeeper would be set with a player's username skin by default, not by UUID... idk...