I think it would be awesome if Druids were able to gain access to an undead form of some kind. So lets give them a "Blight Druid" PrE that does some, if not all, of the following:
1) Adds negative energy spells like Death Aura, Negative Energy Burst and Harm to the druid spell list.
2) Adds some acid spells to the druid spell list.
3) Allows Druids to shapeshift into Zombie and/or Wraith (as in the Palemaster undead form) variants of the Wolf and Bear forms.
4) Adds a stand-alone undead form that increases wisdom and acts similarly to the water and fire elemental forms.