If you find yourself never using a space, give it a new purpose. The builder’s floorplan may name the room one thing but it’s your house, so use it the way you want!
For example, the basement often is an unused abyss of space that we don't utilize because we aren't sure what to do with it. We never go down there except to store unwanted furniture pieces or boxes of the kid's old toys and clothes. A recent trend is converting a basement (or a section of it) into a home theater or game room. This allows the family to have a gathering place and provides a way to maximize their home's square footage. This especially holds true right now with everyone watching their pennies, people are entertaining in the home more than planning nights out on the town.
Another example of unused space is the formal living. Admit it, this space has died out. Gone are the days when we sit with our company in the front room on the good sofa that we forbid the children to touch! For modern families, this room is rarely used and often houses our old furniture that we aren't sure how to use in the rest of the home. So, re-purpose the space! What do you need it to be? What are you missing in the house? Many people turn these rooms into a home office. You can even add french doors to enclose the space if privacy is necessary. The same can be true for formal dining rooms (if you have an eat-in kitchen), spare guest rooms, etc (you see where I am going with this).
My only word of caution is: be mindful of the re-sale value of your home and be sure any changes you make stay in line with the rest of your neighborhood in terms of cost. Nothing is worse than sinking money into making drastic and beautiful changes that you won't earn your money back on.
But overall remember this- transform the house to fit you, don't conform to the house.