Design can be anything you want it to be from something you dream up in your head to something you create as a tangible object. It honestly does not matter what the design even is or if it's realistic because in all design there is something you can take from a idea and use it to create something spectacular. It may not be your first idea that works and is perfect but by the time you think about it and rework it you will have a design that inspires you as well as others. Design is as much about the process and the re-dos than the original idea which means you cant just stop when you get stuck you have to go out and search for more ideas and work those into your design process. This is what it takes to make a good idea turn into a great design.
You can gain inspiration from anything and everything and use that to add new twist to old things as well as design completely new objects and spaces. Your inspiration can be anything from a stick on the floor of a building to the building itself. to design great spaces and things you have to think about what you inspiration meant to the person who designed it. You need to be able to create a story about your inspiration then translate those ideas into a working idea that forces others to wonder what you were thinking about when you designed it.
Personally I am inspired by nature and I choose to combine that with more man made elements like combining a tree stump with white Plexiglas glass to create a juxtaposition of the two elements which separately are beautiful but together create a design that is thought provoking and playful. In my designs I try to get people to wonder why I chose the products I did or why the scale is how it is. This wondering forces others to attempt to interpret my work and in doing so they come up with ideas and inspiration for there own work.
Picture source: http://www.behance.net/gallery/Good-Design-MANIFESTO/268013