I have been living in Japan for the most of the last ten years. Most houses and apartments have a prefab Japanese bath unit. My current place is one of the few that is not one of these units. Our current house in Japan now has exactly what you are describing. It is a wet room, with a door that closes tightly like a shower door. The bath is one of these 2,000 dollar white soak ones we see in the US these days. The bath plug lets out to the floor and the slope is such that it drains down the wet-room's drain. The real missing piece from the pre-fab Japanese units is not so much the drain but the drying unit. I think people forget that when the whole bathroom is a shower the extra moisture that you get from a whole room basically being wet and moist than the amount of moisture you get from an enclosed shower is actually a lot more moisture. In our case even though most of one wall is an open-able window. It still doesn't dry adequately. Unless you are in a desert climate or have someone full time to dry your bath you might factor the dry function in your design. If you look closely at most of the public baths here, their is a wall that opens up completely so that on the day the place is cleaned it is completely dried as well. If you look too closely at some of the less nice public baths you will see mold that looks ancient and will keep you from wanting to take a bath.