For now, the closest you can get to an offline mod is and optimizing your local client settings. Keep an eye on the official Dev Blogs for any news regarding standalone building tools, which remain one of the most requested features by the veteran community.
Many players create a private, password-protected room to simulate a solo experience. This allows you to use the World Editor without interruption, though you are still technically connected to the SGD servers. 3dxchat offline mod