Broader Servant Variety and Customization
I would love to be able to make servants out of creatures and undead. It'd be really cool to have vampirized pet wolves, werewolves, witches, skeletons, um... deer(?) to add to your army.
Additionally, I would like to be able to customize at least the outfits of our regular servants to some degree as I find it a bit jarring to come home to my castle and see a bunch of vampires wearing sun emblems and sun helmets and carrying around sun-styled weaponry and whatnot. I'd even be happy with that stuff just changing automatically to a moon motif when they're turned. I do think the holy light powers should also be changed to have a dark theme after turning, too.