Funny, I've been thinking of this topic recently too. My dream home would have a 3-story kids' playland built with adults in mind. It would sport ball pits that are at least 4' deep, slides up to the ceiling, a network of trampolines encaged in safety nets, an indoor water park, gymnastics equipment like crash pits and tumbling mats, and tons of climbing equipment to include an indoor climbing wall.
But that's all just a lavish dream. Right now if we actually owned the home we're living in, I'd settle for another water heater (my showers always end in a cold spray), new walk-in closets, a finished basement, a bathroom on the main level (it's no fun running upstairs for an emergency tinkle). I'd love it if this home would retain some heat in the wintertime too, and on that note I'd put thermostats in every room. As it is now, there is one thermostat and it's in the living room where it's often hit with direct sunlight, making the thermostat assume that the whole home is just that warm. That does nothing for the rooms upstairs, let me tell you.