I'm the opposite to you here. When I read that a doppleganger is a creature that can take another creature's shape, my inspirational wheels start spinning.
What I thought up, right now as I'm writing this, is that dopplegangers are lone wolves who move like predators in human society. The things that get them going is to imitate and take someone else's place. They don't have personalities of their own, they just take someone else's. If they find someone who they find is interesting they are not above killing that person and taking that person's identity. At certain times they move and at other times they have grand meetings, sages speculate that they move according to the stars.
Maybe not for everyone, but I have a description of a creature that in adventure nr 1 provides for a detective story and in adventure 2 can be much bigger when the dopplegangers of the world unite and throws everything into chaos. Then it surfaces that dopplegangers is a time bomb planted by a vengeful primordial at the moment of the primordial's defeat.