You should try building smaller, that can help you come up with details to fill up the space. Besides, Japanese homes are pretty small and cozy, not big and grandiose. Here are pictures of a Japanese house I built on Bloxburg to spur some ideas. (Of course its not directly from Studio so you may need to do your own furniture.)
Notice the narrowness of the corridors. I also added a shoe rack for the front of the house, considering that the Japanese are all about tidiness.
Here is the traditional living area with tables as if you were to sit on the ground. As the room was already tiny in the first place, there were obvious walls that needed to be filled up such as the one with the picture frame and the drawer. Try to build small so that areas that need filling up with furniture becomes obvious. Also notice the single light right in the centre of the room. Traditional Japanese houses somehow always have a centrepiece lighting and no other extra lighting, making them dimly lit and cozy.
In the bedroom, there is a little shelf with clothing baskets, since, as mentioned before, the Japanese like things nice and tidy. Another thing you may want to consider is colour. Your colour scheme is uniform with a select few colours which is good. however, you may want to change the saturation of these select few colours to be more vibrant. In this bedroom, the brown, yellow and green are rich in colour. Just don’t over do it, otherwise it will start to look cartoonish.
Here is the shower, which is right next to the bedroom. I tried to make a sauna vibe with the wooden plank walls so as to further infuse the Japanese vibe.
Here is the kitchen and living room as well as those Japanese exterior walkways outside of Japanese palace buildings. For the living room, I chose to add a television because while traditional, TVs are starting to become part of the traditional Japanese house vibe. The kitchen does not have any stoves, just a couple of candles to cook things traditionally by fire.
Here is the front of the house. In some Japanese cities like Okinawa, people drove in really tiny cars. They do add to the traditional Japanese vibe.
After a whole day of being cooped up in the house, what better way than to take a stroll in the backyard with greenery and Bonzai trees as well as a pond to enjoy the nice, breezy night.