Well, Arcane Hierophant progresses wild shape even if you don't make it to druid 5. Druid 3/Arcane Hierophant 10 has 13 effective wildshape levels, i.e. tiny to large forms, animals and plants, 13 HD. Not bad. Plenty of good forms. Two more levels would net you huge forms. Huge forms are improvement, but you probably could live without them. As an arcane hierophant, you probably have lots of spells to cast and not much time to rip asses personally. Plus on highest level, there is shapechange. Imho 13 HD should be enough. You could take dragon wildshape and pick some versatile, mobile and durable dragon form; small & medium dragons don't have too many HD anyways.