One of the biggest design is choosing form over function. While the package makes the initial impression, it's the contents that are useful, and your backyard man cave needs to be designed with this in mind. Using your outdoor man cave to showcase collections may impress your friends for a minute, but will ultimately take up space that could be better used for fun and activities or recreation and leisure.
Because the whole family will be subject to your whims, you need to make sure your outdoor man cave functions in such a way that everyone will be able to use it. For instance, let's say you love playing some outdoor poker in the summertime. Maybe that poker table can be covered to serve as your dining table as well. The same goes for your outdoor, all-weather pool table. And if your sports car collection finds a place in your backyard, then be sure the doors stay unlocked for extra seating and pretend driving.
If you decide to really go for it and build an out-building to house your man cave, you may want to consider incorporating some outdoor storage in the mix. In fact, the storage shed you currently have can be re-configured to be a part-time man cave if you're willing to share your space with some lawn equipment.