This is a stunning design and very unique! I don't think I've ever seen a bathroom like this
that wasn't a small enclosed sauna within the bathrrom. I think you could probably add a heater element - perhaps one hidden in a recess with a door that fits flush to the wall or a heat lamp within a different light fixture - and by adding glass doors around the tub you could create a sauna-like space. Or just create a really warm hideaway to enjoy a good book and a long bubble bath!