Project Ara is awesome. The idea of building a phone with all sorts of desired components like a desktop PC makes me happy.
Aside from the critical component modules like the battery, cpu, ram, ect. I would like to see google work with car manufactures to standardize a real physical car key that i would plug into one of my module slots. The key would then be powered and you could use the phone to unlock and start ect. But you could still undock the key if your phone dies. This would eliminate one more thing that i would have to carry around all the time.