Just some ideas I had while playing:
-Add car explosion.
-Add a first person view.
-Add some kind of god mode so that players can fly , that will make it easier to build. It can be activated by typing a code in the chat (something like the creative mode in minecraft).
-Add functional mechanisms such as elevators, lights and eletronic objects.
-Add a gravity gun to lift props.
-Add game modes for public and private servers such as Hide and Seek, Murder and Deathmatch.
I love this game, hope you guys make it even better!