ALPHA PROTOTYPE
My gap is: how to use the space under your bathtub efficiently.
So my selected prototype was the one with sliding doors.
It took me a lot of time to find the materials and to build something similar. With some help of my father (who is a master in everything he does) I built the real prototype.
The doors are sliding easily and it is easy to open them from the bathtub. I had a nice bath yesterday and no water got inside.
You can open 2 doors at the same time and they are made of plastic which is resistant to himidity and which is light and easy to move.
However, instead of wood I would use different material which would last longer in humid places. I would use the same coulour as the bathtub or use some decoration on it.
This door shape was selected not spontaneously. These notches make it easier to open the door from every angle. 
As you can see I have already put some washing powder over there (as I have no special loundry room and my washing machine is in the bathroom too). I was also able to put a big "kind of bowl" ( a plastic washing up bowl) where I put all my dirty clothes to wash them later.
You can't see but the doors are a little bit glossy and can be replaced by different ones (therefore, the user can change the style of the bathroom whenever he wants)