I linked to this over in my Dreadlands thread, but I did come up with a base class: the Shapeshifter.
The Shapeshifter is a spontaneous caster; he uses the Druid spell list and Sorc/Oracle spell progression. However, the focus of the Shapeshifter is... well, you can guess from the name.
I tried to balance how the Shapeshifting power works, and also scale it with level. He can choose domains, but he doesn't get domain powers, or the option of an animal companion. Instead of bonus spells, the domain gives him bonus forms at the respective levels.
You can find him here.
Well, it has the same problem 3.5 shapeshifting did: you can dump Physical ability scores. Pretty easy to optimize at the point.