Could you look at separating walls between the two? What else do you want to put in the bathroom? Probably the best type of tile is a white ceramic tile to blend the floor toilet with the floor. Do users have good vision (not elderly) as if there is a problem of using the floor toilet in the night, you might need a darker tile to delineate the floor toilet

I agree with divadeva. More lights. If you can add pot lights do it. If they're already there add task lighting and up the wattage on the bulbs on the pot lights (replace with LED and you can make them brighter because the wattage on LED is lower for brighter light). If you have all of that add decorative lighting. If ventilation is an issue find a way to move the air around or add air filtration. Maybe the HVAC is deficient. Maybe you can add a HEPA filter instead of the standard one that comes with the unit. Call an HVAC specialist to see if there is a way to boost air flow in the room.

I did a house with nine bathrooms and it was so much fun. For the master bed we had his and hers separate ensuite bathrooms, so hers was feminine and victoruan; it even had a chandelier! His was white with a red shower wall and a blue and white border tile. The counter was tiled with red. It was a very cheerful little bathroom. The daughter's batH was blue and white, the border tiles and washbasin were Chinese themed. A very pretty bathroom. The two guest bathrooms were yellow and cream with hand painted sunflower accent tiles. One powder room was yellow and white, the other was beige with orange and green accents. It had a very whimsical washbasin with a hand painted frog inside. The pool house also has two bathrooms, more basic, mostly cream with wood accents. The colour comes from the stack of towels, red, blue, orange, lime green and yellow.

You can use natural stone like marble for floor but do not use granite for floor at all as it is very slippery when wet. As for tiles do not go for glazed vitrified tiles for the floor for the same reason. ( floor becomes slippery once wet and may cause mishap) Go for ceramic anti skid tiles that are specially meant for bathroom floor. For wall tiles you can use whatever you like and then decide over the matching anti skid floor tile/ marble. Double charged vitrified tiles are usually used for flooring in rest of the house.