An engine that lets you shoot a gun, or move an object, or kick a door! Through an intrusive interface that relies on keypresses for each command. I mean, if you're going to make actions require pressing a button to make a menu appear, you should probably make it a full-out menu, and just show the options that are available (or have a text parser, people like those). Basically useless engine, but bonus star for coming up with a neat demonstration scenario.