Make sure you have at least 1 floor drain. The biggest regret that both my Dad and Grandpa have is that they did not integrate floor drains when they built their garages.
Are you going to heat the garage? If so, I recommend in-floor heating.
Are you going to have an attic? My garage is a 24 x 24 but has an attic above it and I absolutely love it. Next summer I'm adding some support pillars so that I can store even more shit upstairs.
