Think outside the square. Think transformation. Let your imagination go wild and write down all the possible things you could do with an empty room. Let your mind dream up lots of possibilities. Think spa, massage room, meditation area or a personal gym. Some of your ideas may be impractical, but you can cross those out later.
Many modern house plans include extra rooms for non-specific purpose. These can be used by the owner as desired. They may become a home entertainment room, a library or even an exercise room. Check out http://www.coralhomes.com.au/ for examples of home plans.