I would make it a library. Actually, I'd probably take on a roommate, but if that was out -- definitely library.
Basically, I'm with honeybbq - make a room that allows you to pursue your passions whatever that might be.
Art Studio
Home Theater
Sewing Room
Library
Pantry
Boudoir - One of my friends turned a small room into essentially a giant walk in closet complete with vanity/mirror, and a chaise lounge. I think it's bizarre, but she LOVES it.
Hell, I hear rich people have gift wrapping rooms. So really whatever is going to make you smile.