Some ideas I did for my office:

Last time I went on a furniture spree was over 20 years ago, and I scored some great stuff at an auction house that handled a lot of estate sales.
My favorite era for such things, and architecture, is late 19th/early 20th Century, so I have armoires from the 1880s and other stuff from the 1930s-1940s.
Mid-century decor is pretty hot right now, I hear, but I'd check auction houses.
Sometimes just wandering around the stuff clarifies one's vision.

If i was building an office, I'd consider having a space to park a vehicle I love in the office. I would want it in a separate garage bay, but have a glass wall, so the vehicle could be clearly seen from my office. The reason for the separate bay is to keep the odors associated with many antique vehicles out of the office. Having the car there would be a potential conversation piece and it would be close at hand so it could be taken to go pick-up lunch. Of course, the garage space could be used as a place to hang out. If it is large enough, other garage related collectibles, such as motorcycles, bicycles, boats, signs, gas pumps, sports equipment, etc. could be displayed as well.

I would be a bit concerned about having a glass wall on the exterior of the building for security reasons. A glass wall inside a building should be OK.