There's the one space that everybody forgets, which is the ceiling space! Not just building cabinets up to the roof but also to hang things from and really create a "loft" kind of idea. If you're lucky enough to have high ceilings where the garage is, or even on top of, that would be a great...
The portability idea by @Phoenixl37 is genius! It’s important to ensure that if it’s self built that the grounds are stable as well ;if not for working then also for the extra weight for storage. If you are able to get enough supports on a mobile base, you can probably expand on the barn house...