reposting most of what I said in the other thread.
Something we haven't really seen in DR/UT is a proper doppelganger. Not like Ralsei, who simply resembles Asriel, or Gerson(?) which seems to be some sort of necromancy. Think like a doll or drawing of Martlet that looks, talks, and behaves like Martlet to such an extent that she deludes herself into believing she's the real deal, denying her own nature as a darkner.

I'll probably post this in greater detail in the other thread, but the idea of a Darker doppelganger has potential, if not as a secret boss, than as a chapter villain