Custom weapon king is a game where player use and weapon building interface that provides many pre-built primary parts to be attached to each other to create unique weapons. For those who want some more custom they can go into the advanced editor which gives you the ability to place small parts and pieces to build truly unique and successful weapon. It is likely that those who try advanced custom weapons will be dying the moment they use them.
A key component to the game is the complex physics simulation for each part of the gun, leading to the need to adjust and modify constantly till the weapon is in a effective and safe state of usability. The only way to test weapons is on the firing range with various useful equipment and in the line of duty in actual battlefields. A more advanced version of the game can take these ideas and put them into a massively multiplayer environment where the resources to be used in the battle will have to be mined from the earth and refined with machinery of varying levels.
Pushing the limits of this you can bring in the idea of custom buildings and fortresses, custom weapons and tools all with the goal of winning the the war and taking over the planet.