I am interested, though this might be better suited to the Nation section.  Nevertheless, I want in.

I'm pondering a species with caste/hierarchy-based polymorphism.  Like, the grunt-laborers are biologically different from the merchants, are different from the governors, are different from the priest-figures, all tied together with either a Zerg-esque biomass absorbtion, or social mobility actually causing physiological changes in individuals.  It needs work, but I like it.