If you don't need another bathroom, why not turn the empty area under the stairs into a fun area for kids? The coolest part is that you can get away with painting it bubblegum pink and not wonder if it will work with the rest of the house. The kids will love visiting and will remember your house forever. I know this because my aunt had one, and it was so much fun. Plus, we'd keep ourselves entertained for hours on end and the grownups never bothered us.