Found some stuff to post from the WOTC site which ties into doppelgangers, or more accurately, their slightly slower kin, oozes.
http://www.wizards.com/dnd/article.asp? ... 0021227a,0
Ectoplasmic Ooze, an example of a ghost doppelganger perhaps? Or the offsrping of a doppelstadt masquerading as a huanted house?
http://www.wizards.com/dnd/article.asp? ... 0021025a,0
The second spell listed, oozepuppet gives you the power to take control of an ooze. A wizard with this power could try to cause some problems for doppelgangers.
http://www.wizards.com/dnd/article.asp? ... 0021028a,0
Film of Night, a huge shadow ooze
http://www.wizards.com/dnd/article.asp? ... 0021123a,0
At the end of this article is the skitterhaunt and rules for making a skiterhaunt, an ooze residing inside the body of a vermin. Could it be a dungeon ooze, or young ooze who seeks to be a rustic doppelganger, but doesn't want to wait to grow up?
Each of these could represent a new form of doppelganger or a different dynamic of doppelganger development.
I hope that some of this is helpful.