Prototyping in Unity
One of the great thing about Unity is that it allows us to quickly create prototypes of games and flesh out the game mechanics before ever introducing the final art assets to the game. You can essentially make the skeleton code of the game and make it modular enough so that art and other animations can be swapped out on the fly with minimal changes to the code.
I made a prototype of an FPS game for my college senior capstone class exclusively with primitive assets in Unity. The characters were nothing more than basic capsules and the weapon was a rectangle that fired spheres. However, I still was able to get the movement and weapon mechanics to a point where I was satisfied and the game was ready to simply insert some 3D models and animations for the characters and weapons. Even the first maps were made in Unity with primitive 3D objects just so I could get a feel for how the game was going to play.