When I wrote the post on Arcane Texts of the Second Age, I was very tempted to create an NPC in the setting - a wizard who used psychology texts upon human animae in the way that other wizards use tech manuals on techgods.
Then I stopped myself. Yeah. Its a cool idea - but it could be a PC (or a major villain in a campaign). I think that it is easy for setting designers to fall in love with their settings to the point that they need to create virtual PCs. Yes, you need NPCs in a setting - but you need to be careful (1) that they don't overshadow the PCs and (2) that they don't use up cool concepts that should be reserved for PCs.
I'm not sure that the concept I came up for should be a PC, but the thought that it could be gave me pause... and I thought it notable.