The only thing that I liked in BoVD was the cancer mage. I mixed that idea with grafts and came up with pets that can merge with their masters. From leech creatures that act as gills to fungi that cover the skin and provide immunity to (heat, cold, acid, aging, whatever). The difference from grafts (and symbiotes in FF) is the pets can leave their hosts at will (and sometimes at the worst time when harmed or annoyed).